Jak2V617F requires an intact FERM domain for transformation of BaF3.EpoR cells. Parental BaF3.EpoR cells or cells expressing various mutants of Jak2 (as indicated) were used. (A,B) Relative growth of cell lines was determined in a 72-hour culture. Cell growth is shown as a change in cell concentration (A) or was calculated as a percentage compared with cells that were grown in an optimal amount of Epo (B). (C) Cell-cycle distribution was determined in the absence of growth factor (typical experiment). (D) Cells expressing Jak2V617F were used and compared with cells expressing Jak2V617F.Y114A in the absence of growth factors. Cellular proteins were detected by immunoblotting as indicated.