Jak2V617F and Stat5 are constitutively activated in vivo. (A) Immunoprecipitation and Western blot analysis in GFP-sorted, Jak2V617F, Jak2wt, and empty vector transfected Ba/F3 cells showing constitutively active Jak2 and Stat5 after 4 hours of starvation in 1% bovine serum albumin in Jak2V617F stable Ba/F3 cells. (B) Phospho-flow analysis showing significant increases in phosphorylated Stat5 in the BM and spleens of Jak2V617F mice compared with Jak2wt mice, as indicated by the significant shift to the left (P < .001).