DiagnosisClinical DiagnosisAniridia is characterized by complete or partial iris hypoplasia with associated foveal hypoplasia resulting in reduced visual acuity and nystagmus presenting in early infancy. Frequently associated ocular abnormalities, often of later onset, include cataract, glaucoma, and corneal opacification and vascularization.Techniques used to identify the ocular abnormalities of aniridiaSlit lamp examination. Partial or complete iris absence, iris translucency, or abnormal architecture and pupillary abnormalities may be seen; corneal opacification and vascularization, cataract, and glaucoma can also be detected if present.Iris fluorescein angiography may identify subtle iris hypoplasia but is rarely used clinically.Optical coherence tomography (OCT) may be used to document foveal hypoplasia in atypical cases. Although OCT is difficult to perform in the presence of nystagmus, useful images can be obtained with persistence.High-frequency ultrasound biomicroscopy (UBM). In infants with corneal opacity or severe corneal edema resulting from associated congenital glaucoma, high-frequency anterior segment ultrasound examination can demonstrate iris hypoplasia and/or absence [Nischal 2007].Aniridia may occur as one of the following:Isolated aniridia without systemic involvement caused by mutation of PAX6 or deletion of a regulatory region controlling PAX6 expression
Note: Isolated aniridia may occur in individuals with a positive family history consistent with autosomal dominant inheritance (familial aniridia: 70% of all individuals with aniridia) and in individuals with no family history of aniridia (simplex aniridia, commonly referred to as "sporadic aniridia": 30% of individuals with aniridia) [Valenzuela & Cline 2004].
ORThe Wilms tumor-aniridia-genital anomalies-retardation (WAGR) syndrome WAGR syndrome may be diagnosed on the following findings:A visible deletion of 11p13 found on cytogenetic testing
ORA submicroscopic deletion involving the PAX6 (aniridia) locus and the adjacent WT1 (Wilms tumor) locus found on FISH testing or heterozygosity testing
OROne or more additional findings of WAGR syndrome found on physical examination in individuals with aniridia
Note: (1) Because Wilms tumor, intellectual disability, and behavioral abnormalities are unlikely to be evident in a very young child with WAGR syndrome, the clinical diagnosis of WAGR syndrome usually cannot be established or ruled out until a child has passed through the age of risk for these manifestations. (2) The external genitalia are usually normal in females with WAGR syndrome [Fischbach et al 2005].TestingCytogenetic testing. High-resolution cytogenetic testing at the 600-650-band level detects deletions involving 11p13 in up to 20% of individuals with no family history of aniridia.Molecular Genetic TestingGenesMutations or deletions in PAX6 or its control elements are associated with isolated aniridia.Contiguous gene deletions including PAX6 and WT1 are associated with aniridia and the risk of one or more additional manifestations of WAGR.Clinical testingWAGR syndromeFISH testing. A probe or probes spanning PAX6, WT1, the regions flanking PAX6, and the intervening sequence between PAX6 and WT1 can be used:To detect cryptic deletions and contiguous gene deletions in individuals with no family history of aniridia and normal cytogenetic studies
ANDTo confirm, when necessary, deletion of WT1 in individuals with a cytogenetic abnormality involving 11p13 [Crolla & van Heyningen 2002].Deletion testing. Using a variety of methods including quantitative polymerase chain reaction (qPCR)/real-time PCR (rt-PCR), and multiplex ligation-dependent probe amplification (MLPA), deletions of PAX6 and WT1 can be identified in individuals with WAGR syndrome.Isolated aniridiaSequence analysis. Sequence analysis can be used to identify mutations in or near PAX6 associated with isolated aniridia [Robinson et al 2008].Deletion testing. Using a variety of methods including qPCR/rt-PCR, and MLPA, exonic or whole-gene deletions of PAX6 can be detected in 39% of individuals with isolated aniridia [Robinson et al 2008].Table 1. Summary of Genetic Testing of Aniridia by Phenotype and Family HistoryView in own windowPhenotypeGene SymbolTest MethodMutations DetectedMutation Detection Frequency by Phenotype and Test MethodTest AvailabilityFamily HistoryNegativePositiveWAGR syndrome 1PAX6 and contiguous genesHigh-resolution cytogenetic testingCytogenetic deletion 11p1357%NAClinicalPAX6 and WT1FISHSubmicroscopic deletion14%NAClinicalDeletion testing ^{2Whole-gene deletionsUnknownNAIsolated aniridia 3 PAX6Sequence analysis of coding regionSequence alterations55%62.5%ClinicalDeletion testing 2Exonic deletions and deletions of control regions22%17%NA = not applicable1. Wilms tumor-aniridia-genital anomalies-retardation syndrome. Note: In young individuals, Wilms tumor and intellectual disability may not be evident; in females, external genitalia are often normal.2. Isolated aniridia: aniridia without systemic involvement3. Testing that identifies deletions/duplications not detectable by sequence analysis of genomic DNA; a variety of methods including qPCR, rtPCR, MLPA, and array CGH may be used.Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.Testing StrategyEstablishing the diagnosis of isolated aniridia vs WAGREvaluate the proband with cytogenetic, FISH testing, and/or deletion testing of PAX6 and WT1 first when the proband is:An infant with aniridia who is a simplex case (i.e., a single occurrence in the family)
ORAn older individual with aniridia and intellectual disability and/or Wilms tumor and/or genital anomalies.Identification of deletion of PAX6 and WT1 by cytogenetic studies, FISH testing, or deletion testing confirms the diagnosis of WAGR syndrome.Evaluate the proband with PAX6 sequence analysis and/or PAX6 deletion testing first when the proband:Is known to have isolated aniridia (either because s/he has exceeded the age of risk for Wilms tumor and intellectual disability or s/he has a positive family history of isolated aniridia)
ORHas no family history of aniridia, a normal karyotype, and no deletion of WT1 by FISH testing or deletion testing.Identification of a PAX6 sequence alteration, a PAX6 exonic deletion, or deletions of PAX6 control regions confirms the diagnosis of isolated aniridia.Prenatal diagnosis and preimplantation genetic diagnosis (PGD) for at-risk pregnancies require prior identification of the disease-causing mutation in the family.Genetically Related (Allelic) DisordersPAX6} mutations. Missense mutations with residual protein function produce alternative ocular and sometimes neurodevelopmental phenotypes detailed in Table 2 [Prosser & van Heyningen 1998, Hanson et al 1999, Azuma et al 2003, Vincent et al 2003, Dansault et al 2007]. All are inherited in an autosomal dominant manner.Table 2. Other Ocular Phenotypes Caused by PAX6 MutationsView in own windowOcular PhenotypeManifestationsKeratitisLimbal stem cell deficiency with vascularization and opacification of the cornea ± foveal hypoplasiaPeters anomaly ^{1Central corneal opacity caused by iridocorneal adhesions or lenticulocorneal adhesions. Glaucoma in 50%Ectopia pupillaePupil displaced from center of irisJuvenile cataractsEarly-onset lens opacitiesIsolated foveal hypoplasiaNormal iris, reduced foveal reflex, reduced macular pigmentation, retinal vessels crossing the usually avascular foveal zoneOptic nerve aplasia/hypoplasia or colobomaSmall, absent, or malformed optic nerve headsMicrophthalmia, cataract & nystagmusVery small eye, early lens opacities, glaucoma commonFoveal hypoplasia/macular coloboma with neurodevelopmental anomaliesAbsent or highly malformed central chorioretinal area, variable neurologic abnormalities (e.g., cerebellar syndrome, cortical atrophy, low IQ, absent pineal gland)From pax6​.hgu.mrc.ac.uk1. PAX6 mutations have not been detected in most individuals with Peters anomaly [Churchill et al 1998, Chavarria-Soley et al 2006, Dansault et al 2007].Individuals with two PAX6 loss-of-function mutations. In the rare cases of a homozygous PAX6 mutation, severe craniofacial abnormalities, anophthalmia, absent or malformed nose, absent adrenal glands, central nervous system malformations, and fetal or neonatal death have occurred [Hodgson & Saunders 1980, Glaser et al 1994].}

Natural HistoryAniridia is a panocular disorder affecting the cornea, iris, intraocular pressure, lens, fovea, and optic nerve. The phenotype is variable between and within families; however, affected individuals usually show little variability between the two eyes. Individuals with aniridia characteristically show nystagmus, impaired visual acuity (usually 20/100 - 20/200), and foveal hypoplasia. Other abnormalities include corneal changes, glaucoma, cataract, lens subluxation, strabismus, and optic nerve coloboma and hypoplasia.The reduction in visual acuity is primarily caused by foveal hypoplasia, but cataracts, glaucoma, and corneal opacification are responsible for progressive visual failure. Most children with aniridia present at birth with an obvious iris or pupillary abnormality or in infancy with nystagmus (usually apparent by six weeks of age). Congenital glaucoma rarely occurs in aniridia; in such cases, a large corneal diameter and corneal edema may be the presenting findings. Despite their many ocular problems, most individuals with aniridia can retain useful vision with appropriate ophthalmologic management.Iris. The most obvious ocular abnormality is iris hypoplasia. The severity varies from a nearly normal iris to almost complete iris absence in which a small stump of residual iris tissue is visible on gonioscopy or ultrasound biomicroscopy [Okamoto et al 2004]. In less extreme cases, the pupil size may be normal, but there may be loss of the iris surface architecture or the presence of iris transillumination. Other iris changes include partial iris defects (resembling a coloboma) or eccentric or misshapen pupils and iris ectropion [Nelson et al 1984, Willcock et al 2006].Lens. Congenital lens opacities (especially polar) are common. Often there is persistent vascularization of the anterior lens capsule (tunica vasculosa lentis) or remnants of the pupillary membrane. The lens opacities are rarely dense enough to require lens extraction in infancy, but visually significant lens opacities eventually develop in 50%-85% of affected individuals, often in the teens or early adulthood. Lens subluxation or dislocation occurs but is uncommon.Intraocular pressure. When elevated intraocular pressure is associated with loss of retinal ganglion cells resulting in visual field loss and optic nerve cupping, a diagnosis of glaucoma is made. Both elevated intraocular pressure and glaucoma are common in aniridia, but the exact prevalence is unknown. The onset of glaucoma is usually in later childhood or adulthood; glaucoma in infancy is rare.Cornea. Keratopathy (corneal degeneration) is a relatively late manifestation with multifactorial causes including limbal stem cell abnormalities [Ramaesh et al 2005]. Changes vary from mild peripheral vascularization to pancorneal vascularization, opacification, and keratinization. Inadequate tear production is common and exacerbates the ocular surface problems. Central corneal thickness is increased – a finding of uncertain clinical relevance, but which may result in undermeasurement of intraocular pressure on tonometry [Brandt et al 2004, Whitson et al 2005].Fovea. Foveal hypoplasia is usually present. Findings include reduced foveal reflex, macular hypopigmentation, and crossing of the usual foveal avascular zone by retinal vessels.Optic nerve. Optic nerve hypoplasia (i.e., the optic nerve head appears abnormally small) may occur in up to 10% [McCulley et al 2005].Aniridic fibrosis syndrome. Patients with aniridia with a history of multiple ocular procedures (penetrating keratoplasty, intraocular lenses (IOLs), and drainage tube insertion) may develop aniridic fibrosis syndrome in which a fibrotic retrolenticular and retrocorneal membrane arises from the root of the rudimentary iris tissue. This membrane may cause forward displacement of the IOLs, IOL entrapment, and corneal decompensation [Tsai et al 2005].Central nervous system. Individuals with isolated aniridia may show reduced olfaction and cognition, behavioral problems, or developmental delay. Central nervous system abnormalities (including absence or hypoplasia of the anterior commissure, abnormalities of grey matter in the anterior cingulate cortex, cerebellum, temporal and occipital lobes, white matter deficits in and reduced volume of the corpus callosum, absence of the pineal gland, and occasionally olfactory bulb hypoplasia) can be demonstrated on MRI [Sisodiya et al 2001, Free et al 2003, Mitchell et al 2003, Ellison-Wright et al 2004, Valenzuela & Cline 2004, Bamiou et al 2007].Hearing. Central auditory processing difficulties (from abnormal interhemispheric transfer) present in some individuals may cause hearing difficulties. This finding is particularly important in the context of associated visual impairment [Bamiou et al 2007].WAGR syndrome. Individuals with cytogenetically visible deletions of 11p13 or cryptic deletions of PAX6 and WT1 may develop WAGR syndrome: wilms tumor-aniridia-genital anomalies-retardation syndrome [Fischbach et al 2005]:Wilms tumor risk for individuals with a cytogenetically visible deletion of 11p13 or a submicroscopic deletion that involves PAX6 and WT1 is probably as high as 50%. Individuals with WAGR syndrome are more likely than those with isolated Wilms tumor to develop bilateral tumors and have an earlier age of diagnosis and a more favorable tumor histology with better prognosis.Aniridia is almost universally present in individuals with such a deletion and typically is severe. However, WAGR without aniridia has been described.Genitourinary abnormalities include cryptorchidism (most commonly, in 60% of males), uterine abnormalities, hypospadias, ambiguous genitalia, streak ovaries, urethral strictures, ureteric abnormalities, and gonadoblastoma.Intellectual disability and behavioral abnormalities in WAGR syndrome are highly variable:Seventy percent of individuals with WAGR syndrome have intellectual disability (defined as IQ <74); other individuals with WAGR syndrome can have normal intellect without behavior problems.Behavioral abnormalities include attention deficit hyperactivity disorder (ADHD), autism spectrum disorders (see Autism Overview), anxiety, depression, and obsessive compulsive disorder.Neurologic abnormalities occur in up to one-third of individuals with WAGR syndrome. Findings include hypertonia or hypotonia, epilepsy, enlarged ventricles, corpus callosum agenesis, and microcephaly.End-stage renal disease (ESRD). The risk of later ESRD is significant, relating to Wilms tumor and its surgery, focal segmental glomerulosclerosis, and occasionally renal malformation. The rate of ESRD is 36% with unilateral Wilms tumor and 90% with bilateral Wilms tumor. Approximately 25% of individuals with WAGR syndrome have proteinuria ranging from minimal to overt nephritic syndrome [Breslow et al 2005, Fischbach et al 2005].Obesity. The association of obesity in the WAGR spectrum, for which the acronym WAGRO has been suggested, has been confirmed [Brémond-Gignac et al 2005a].Affected individuals may also show craniofacial dysmorphism, hemihypertrophy, growth retardation, scoliosis, and kyphosis. Other anomalies reported on occasion include polydactyly and congenital diaphragmatic hernia [Nelson et al 1984, Brémond-Gignac et al 2005b, Manoukian et al 2005, Scott et al 2005] (see Congenital Diaphragmatic Hernia Overview).Early studies recognized that 30% of individuals with aniridia with no family history of aniridia developed Wilms tumor within the first five years of life; subsequent studies revealed that the risk may be lower [Gronskov et al 2001]. It is now known that these individuals have WAGR syndrome caused by a contiguous gene deletion encompassing both PAX6 and the nearby Wilms tumor suppressor gene (WT1). Absence of one WT1 allele in the germline in these individuals leads to a high risk (~45%) of Wilms tumor occurring through somatic mutation that results in loss of heterozygosity (LOH) in a single differentiating kidney cell.

Genotype-Phenotype CorrelationsIsolated aniridia. PAX6 missense mutations (often in the paired domain) tend to produce atypical or variable-phenotype aniridia or related disorders (see Table 2) such as foveal hypoplasia, autosomal dominant keratitis, developmental abnormalities of the optic nerve, and Peters anomaly, sometimes associated with neurodevelopmental abnormalities.PAX6 haploinsufficiency through loss-of-function intragenic mutations (often premature termination codons), larger deletions, or occasional chromosomal rearrangements at nearby regulatory elements produces classic aniridia [Kleinjan & van Heyningen 1998, Prosser & van Heyningen 1998, Gronskov et al 1999, Hanson et al 1999, Lauderdale et al 2000, van Heyningen & Williamson 2002, Chao et al 2003, Tzoulaki et al 2005, Dansault et al 2007].Although the phenotype can be variable within a family, individuals usually show little difference between the two eyes. The causes for this variation in phenotype among individuals with the same mutation are unknown [Negishi et al 1999].WAGR syndrome is caused by either cryptic or cytogenetically visible deletions involving varying amounts of 11p that include band 11p13 with PAX6 and neighboring genes. The loss of WT1 produces genitourinary and renal abnormalities and predisposes to Wilms tumor, which results from LOH. Deletion of one copy of PAX6 causes aniridia. The exact gene loss responsible for intellectual disability is uncertain [Fischbach et al 2005].

Differential DiagnosisRieger anomaly, a form of anterior segment mesenchymal dysgenesis, is characterized by severe iris atrophy, corectopia (displaced pupils), iris holes, and, frequently, childhood-onset glaucoma. Rieger anomaly may be distinguished from aniridia by the presence of posterior embryotoxon (visible Schwalbe's line seen as a white line just inside the corneal limbus) with attached iris strands, relatively good visual acuity, and the absence of nystagmus or foveal abnormality.Iris coloboma is a developmental defect resulting in a focal absence of the iris and a keyhole-shaped pupil; the rest of the iris is normal. Chorioretinal coloboma may be associated. Most iris colobomas are not associated with reduced visual acuity or nystagmus unless accompanied by a large posterior coloboma that involves the optic nerve and fovea; such large chorioretinal colobomas are apparent on fundoscopic examination.Gillespie syndrome, characterized by partial iris hypoplasia, cerebellar ataxia, and intellectual disability, can be distinguished from aniridia by a characteristic iris configuration in Gillespie syndrome showing a scalloped pupillary edge with iris strands extending onto the anterior lens surface [Nelson et al 1997].Oculocutaneous albinism (OCA) and ocular albinism typically present in early infancy with nystagmus but a structurally complete iris, typical diffuse iris transillumination (resulting from reduced pigment in the iris pigment epithelium), hypopigmented fundus, and, in the case of OCA, skin and hair hypopigmentation, which distinguish these disorders from aniridia (see Oculocutaneous Albinism Type 1, Oculocutaneous Albinism Type 2, Oculocutaneous Albinism Type 4, and X-Linked Ocular Albinism).The other causes of nystagmus and poor vision in infancy (e.g., retinal dysplasia, retinal dystrophy, congenital cataracts, optic nerve hypoplasia, congenital infections) lack the iris changes seen in aniridia.Causes of partial or complete absence of iris tissue in adults include trauma, prior ocular surgery, and the iridocornealendothelial (ICE) syndromes. The age at onset, medical history, and lack of other ocular features of aniridia should prevent diagnostic confusion with aniridia.Note to clinicians: For a patient-specific ‘simultaneous consult’ related to this disorder, go to , an interactive diagnostic decision support software tool that provides differential diagnoses based on patient findings (registration or institutional access required).

ManagementEvaluation Following Initial DiagnosisTo establish the extent of disease in an individual diagnosed with aniridia, the following are recommended:Evaluation of visual acuity (not easily performed in infants), the degree of iris tissue deficiency, the presence of foveal and optic nerve hypoplasia in order to predict future visual functionEvaluation for the presence and degree of corneal involvement, cataract, and glaucoma, as they are potentially treatable causes of further visual reduction; however, they may not appear until later in life.Treatment of ManifestationsAniridia. During childhood, simple measures are often the most important:Regular eye examinations and correction of refractive errors. Refractive errors range from high myopia through emmetropia to high hypermetropia. Spectacle correction of refractive errors is usually recommended as use of contact lenses can be difficult in the presence of keratopathy and reduced tear production.Tinted or photochromic lenses to reduce light sensitivity associated with the large pupillary apertureOcclusion therapy for anisometropic amblyopia or strabismic amblyopiaOptical low-vision aids and other devices such as closed-circuit television systems to help adults and children of school ageAdvice and help with schoolingSocial supportLens. Cataract extraction can significantly improve visual acuity in those patients with severe lens opacities. It should be remembered that in aniridia visual improvement after surgery is limited by foveal hypoplasia; thus, mild to moderate lens opacities may not require surgery:Children rarely require surgery (lens aspiration or lensectomy).In adults, phacoemulsification can be successful.Note: (1) A significant number of individuals with aniridia have poor zonular stability, which increases the risk for intraoperative complications and influences the choice of surgical technique and options for IOL implantation [Schneider et al 2003]. (2) The use of various types of black diaphragm aniridic IOLs may reduce glare or light sensitivity but may be associated with a slightly higher rate of surgical complications [Reinhard et al 2000, Menezo et al 2005, Pozdeyeva et al 2005].Intraocular pressureGlaucoma is initially treated with topical anti-glaucoma medication.Surgery is reserved for eyes that do not respond to medical therapy:Trabeculectomy with or without antimetabolites (e.g., 5-fluorouracil, mitomycin C) is the operation of choice.Drainage tube surgery (with or without antimetabolites) or cyclodiode laser treatment may be necessary in refractory cases [Khaw 2002, Kirwan et al 2002, Arroyave et al 2003].Note: (1) Glaucoma presenting in infancy is more difficult to treat. Medical treatment is generally ineffective and surgery is required. Goniotomy and trabeculotomy have a low success rate, but trabeculectomy with or without antimetabolites is often successful [Nelson et al 1984, Okada et al 2000, Khaw 2002]. (2) While goniosurgery has been suggested as a preventive measure, glaucoma never develops in most of those with aniridia [Swanner et al 2004].CorneaOcular surface disease can be treated medically using lubricants, mucolytics, and punctal occlusion. Note: Drops without preservatives are often required to avoid preservative-related ocular surface toxicity.When corneal opacification causes significant visual reduction, penetrating keratoplasty (PK) may be considered; however, in the presence of the significant limbal stem cell deficiency observed in aniridia, PK alone has a poor prognosis [Tiller et al 2003].Aniridic fibrosis syndrome. Surgical intervention is recommended at the first sign of aniridic fibrosis syndrome [Tsai et al 2005].Wilms tumor. See Wilms Tumor Overview.SurveillanceGlaucoma. Individuals with aniridia should undergo annual glaucoma screening throughout life including:Measurement of intraocular pressureOptic disc examinationVisual field assessment, when possibleNote: Assessment of the optic disc and visual field may be difficult in the presence of media opacities and nystagmus.Aniridic fibrosis syndrome. Patients with aniridia with a history of multiple ocular procedures (penetrating keratoplasty, IOLs, and drainage tube insertion) should be monitored for aniridic fibrosis syndrome [Tsai et al 2005].Wilms tumor. Children with aniridia and a WT1 deletion require regular renal ultrasound examinations every three months and follow-up by a pediatric oncologist until they reach age eight years. See Wilms Tumor Overview. (Those without deletion of the WT1 locus are at very low risk for Wilms tumor and do not require such screening [Gronskov et al 2001, Muto et al 2002].)Renal function. Because of the increased risk of renal impairment in WAGR syndrome, it has been suggested that renal function be evaluated lifelong in those with WAGR syndrome, especially those with bilateral Wilms tumor [Breslow et al 2005].Hearing. Children with WAGR syndrome and isolated aniridia may have abnormal hearing despite a normal audiogram; thus, detailed audiologic evaluation is recommended [Bamiou et al 2007].Evaluation of Relatives at RiskIt is recommended that offspring and sibs of individuals with aniridia have an eye examination in infancy.See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.Therapies Under InvestigationSearch ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions. Note: There may not be clinical trials for this disorder.

Molecular GeneticsInformation in the Molecular Genetics and OMIM tables may differ from that elsewhere in the GeneReview: tables may contain more recent information. —ED.Table A. Aniridia: Genes and DatabasesView in own windowGene SymbolChromosomal LocusProtein NameLocus SpecificHGMDPAX611p13Paired box protein Pax-6PAX6 @ The Human Genetics Unit Edinburgh U.K.PAX6Data are compiled from the following standard references: gene symbol from HGNC; chromosomal locus, locus name, critical region, complementation group from OMIM; protein name from UniProt. For a description of databases (Locus Specific, HGMD) to which links are provided, click here.Table B. OMIM Entries for Aniridia (View All in OMIM) View in own window 106210ANIRIDIA; AN 194070WILMS TUMOR 1; WT1 194072WILMS TUMOR, ANIRIDIA, GENITOURINARY ANOMALIES, AND MENTAL RETARDATION SYNDROME; WAGR 607102WT1 GENE; WT1 607108PAIRED BOX GENE 6; PAX6Molecular Genetic PathogenesisPAX6 belongs to the PAX (paired box) family of genes that code for highly conserved DNA-binding proteins believed to be important in controlling organogenesis by altering expression of other genes [van Heyningen & Williamson 2002]. PAX6 is expressed in ocular, neural, nasal, and pancreatic tissue during development. Heterozygous mutations of PAX6 appear to disturb ocular morphogenesis, resulting in aniridia and related ocular phenotypes, and also may produce mild central nervous system defects [Sisodiya et al 2001, Free et al 2003, Ellison-Wright et al 2004, Valenzuela & Cline 2004]. Homozygous loss of PAX6 function leads to anophthalmia and central nervous system defects and is fatal [Hodgson & Saunders 1980, Glaser et al 1994].Normal allelic variants. PAX6 occupies 22 kb on chromosome 11p13 and contains 14 exons and 13 introns. The genomic sequence is available (www.sanger.ac.uk). Five non-pathogenic normal allelic variants are known.Pathologic allelic variants. More than 300 PAX6 mutations have been identified; 286 are associated with congenital eye malformations [Prosser & van Heyningen 1998, Tzoulaki et al 2005]:Approximately 72% are intragenic loss-of-function mutations that introduce a premature termination codon into the PAX6 open reading frame and (occasionally) mutations of up- or downstream regulatory sequences [Prosser & van Heyningen 1998, Crolla & van Heyningen 2002, Tzoulaki et al 2005].Approximately 12% are missense mutations that have been detected in typical aniridia and that may code for near-to-loss-of-function protein [Azuma et al 1998, Vincent et al 2003, Chauhan et al 2004, Tzoulaki et al 2005].Four CpG dinucleotides in exons 8, 9, 10, and 11 are the most common mutation sites, accounting for 21% of all reported mutations [Tzoulaki et al 2005]. Large deletions that may involve other genes (e.g., WT1) also produce aniridia.Many mutations have been reported in PAX6, both in aniridia and related ocular phenotypes such as Peters anomaly, foveal hypoplasia, and optic nerve anomalies:Of the 257 PAX6 mutations known to cause aniridia, most lead to loss of protein function and comprise nonsense mutations (39%), splice mutations (13%), frameshifting deletions and insertions (25%), inframe insertions and deletions (6%), missense mutations (12%), and run-on mutations (5%) [Prosser & van Heyningen 1998, Tzoulaki et al 2005].Of the 29 known mutations for non-aniridia eye disorders, 69% are missense mutations [Tzoulaki et al 2005].Normal gene product. PAX6 encodes the PAX6 protein, a 422-amino acid protein that acts as a transcription factor. PAX6 contains a paired domain and a paired-type homeodomain, both with DNA-binding capability, separated by a lysine-rich linker region. A C-terminal proline, serine, and threonine-rich (PST) domain acts as a transcriptional activator. PAX6 protein is thought to act as the major controller of ocular development during embryogenesis by effects on cellular proliferation, differentiation, migration, and adhesion; several target genes have been identified [van Heyningen & Williamson 2002]. PAX6 protein expression continues in the adult retina, lens, and cornea and may help maintain good ocular health [Koroma et al 1997, van Heyningen & Williamson 2002].Various isoforms of PAX6 protein are derived through alternative splicing (PAX6-ex12, PAX6-5a,6', PAX6-5a). The ratios of these isoforms may be critical to normal ocular development [Singh et al 2002].Abnormal gene product. Most PAX6 mutations cause loss of protein function. This was previously believed to occur primarily through premature protein truncation but is now hypothesized to arise from nonsense-mediated decay [Prosser & van Heyningen 1998, Tzoulaki et al 2005]. Missense mutations are believed to produce reduced-function protein, resulting in the variant ocular phenotypes or (if protein function is greatly reduced) in aniridia. Reduction of expression of alternatively spliced PAX6 protein isoforms can also cause an altered or less severe phenotype [Azuma et al 1999, Vincent et al 2003, Chauhan et al 2004].