Reduction of Stat5 gene dosage impairs JAK2^V617F-induced polycythemia in vivo. Hematocrit (A), reticulocyte counts (B), and leukocyte counts (C) from PB of recipients of BM from Stat5a/b^fl/+ and Stat5a/b^fl/− donors transduced with JAK2^V617FMIGFP or MIGFPCre retrovirus analyzed on day 100 after transplantation. Hematocrit was significantly lower in the Stat5a/b^fl/−JAK2^V617FMIGFPCre group (P = .0114 vs Stat5a/b^fl/−JAK2^V617FMIGFP by t test).