Subjects will initiate study medication at an oral dose of 50 mg/day (25 mg/day for East Asians), which will be increased or decreased as clinically indicated to the lowest dose that maintains a stable platelet count greater than or equal to 20,000/microL above baseline while maximizing tolerability. Platelet treatment response is defined as platelet count increases to 20,000/microL above baseline at three months, or stable platelet counts with transfusion independence for a minimum of 8 weeks. Erythroid response for subjects with a pretreatment hemoglobin of less than 9 g/dL will be defined as an increase in hemoglobin by greater than or equal to 1.5g/dL without packed red blood cell (PRBC) transfusion support, or a reduction in the units of transfusions by an absolute number of at least 4 PRBC transfusions for eight consecutive weeks compared with the pretreatment transfusion number in the previous 8 weeks. Neutrophil response will be defined in those with a pretreatment absolute neutrophil count (ANC) of less than 0.5 times 10(9)/L as at least a 100 percent increase or an absolute increase greater than 0.5 times 10(9)/L. Subjects with a platelet, erythroid, and/or neutrophil response at 12 weeks may continue study medication (extended access) until they meet an off study criteria. Subjects with platelet, erythroid, or neutrophil response at 12 weeks may continue study medication for an additional 4 weeks (to ensure eligibility) prior to being consented for entry into the extended access part of the trial. Patients may remain on the extended access until they met an off study criteria.