FGR and polycythemia: Although Src kinases are activated by JAK2 in erythroid cells upon Epo stimulation [28] and a Src inhibitor impaired Epo-independent maturation of PV erythroid progenitors [13], we demonstrated that polycythemia induced by JAK2 V617F was independent of Lyn, Hck, and Fgr, the three principal Src family kinases expressed in myeloerythroid cells.