EPAS1 and polycythemia: The colony-forming unit (CFU) assay of bone marrow hematopoietic progenitors revealed increased erythroid colony number in mutant mice (Figure 5C,D), indicating that the gain-of-function mutant HIF2α increased the erythroid differentiation of the progenitor cells and further supporting an EPO-independent component of polycythemia in this syndrome [19].