Retroviral expression of Stat5a complements the deficiency of Stat5 in inducing PV by Jak2V617F. (A) Experimental design for rescue of Jak2V617F-induced PV by retroviral transduction/transplantation of Stat5a in the BM of Stat5-deficient Jak2V617F-expressing (MxCre;Jak2V617F/+;Stat5fl/fl) mice. (B) Immunoblot analysis of Stat5 protein levels in leukocytes 4 weeks after transplantation. Erk2 was used as a loading control. Peripheral blood HCT (C), Hb (D), and RBCs (E) were significantly increased in recipients receiving Stat5a-transduced BM (n = 4). Blood counts were measured at 4, 8, and 12 weeks after transplantation. Data are represented as mean ± SEM. Spleen weight (F) and Epo-independent CFU-E colonies (G) were also markedly elevated in mice receiving Stat5a-transduced BM compared with those receiving vector-transduced BM. Data are presented as mean ± SEM. Asterisks show significant differences by t test (**P < .005). (H) Flow cytometric analysis demonstrates a marked increase in CD71⁺Ter119⁺ erythroid precursors in the spleens of recipient mice transplanted with Stat5a-transduced BM compared with those with vector-transduced BM. Shown are representative dot plots from 3 independent experiments.