Efficient deletion of Stat5 by Mx-Cre abolishes polycythemia and reticulocytosis induced by JAK2^V617F. Hematocrit (A) and reticulocyte counts (B) of untreated (−) or pIpC-treated (+) recipients of JAK2^V617FMIGFP-transduced BM from Mx-Cre;Stat5a/b^fl/+ (left) or Mx-Cre;Stat5a/b^fl/− (right) donors on day 60 after transplantation. Both values were significantly lower in the Stat5a/b^fl/− (+pIpC) group (P < .0001 vs Stat5a/b^fl/− [−pIpC] by t test). (C) Flow cytometric plots of GFP expression in PB leukocytes from 3 representative pIpC-treated recipients of JAK2^V617FMIGFP-transduced BM from Stat5a/b^fl/− donors, with GFP⁺ populations ranging from 15%-90%. (D) Spleen weights from the mice in panel A at time of autopsy on day 100. (E) Western blot analysis of primary BM myeloerythroid cell extracts from representative untreated (−) or pIpC-treated (+) recipients of JAK2^V617FMIGFP-transduced BM from Mx-Cre;Stat5a/b^fl/+ or Mx-Cre;Stat5a/b^fl/− donors from panel A. As controls, protein extracts from parental and JAK2^V617F-expressing Ba/F3 cell lines and BM from a normal mouse were used. Cell extracts were immunoblotted with the indicated Abs against total or phosphorylated STAT5 and ERK1/2 and against total JAK2 and BCL-X. An anti-eIF4e immunoblot demonstrating equivalent protein loading is shown at the bottom. Note that some STAT5 protein is detectable in pIpC-treated recipients because of contamination with cells of host origin.