Stuhrmann et al. (2001) reported a Bedouin Arab family consisting of 4 brothers, aged 4 to 8 years, who had either sustained markedly elevated (greater than 1,000 x 10(9) per liter) or moderately elevated (greater than 500 x ... Stuhrmann et al. (2001) reported a Bedouin Arab family consisting of 4 brothers, aged 4 to 8 years, who had either sustained markedly elevated (greater than 1,000 x 10(9) per liter) or moderately elevated (greater than 500 x 10(9) per liter) platelet counts, 2 healthy sisters, and their first-cousin parents, who had normal platelet counts. The 4 brothers with familial thrombocytosis had normal plasma thrombopoietin levels and did not present with any thrombotic or hemorrhagic complications. The intron 3 mutation in thrombopoietin (THPO; 600044.0001) was not identified in any of the sibs. In addition, segregation analysis using a polymorphic CA marker revealed completely discordant THPO alleles among the affected brothers. In the 2 children with the highest platelet counts, minimal liver enlargement and mild to moderate splenomegaly was manifest.