To investigate the role of Src kinases in the polycythemia induced by JAK2 V617F, we employed donor mice lacking Lyn, Hck, and Fgr, the three principal Src kinases in myeloerythroid progenitor cells [22], [29], as BM donors for transduction with JAK2 V617F. The gene discussed is FGR; the disease is polycythemia.