Mice expressing Jak2V617F develop MPN. Peripheral blood hematocrit (A), hemoglobin (B), and RBC (C) counts were significantly increased in heterozygous and homozygous Jak2V617F mice compared with controls (V617F/+). Mean corpuscular volume (MCV; D) was significantly reduced in both heterozygous and homozygous Jak2V617F mice compared with controls (V617F/+). WBC (E), neutrophil (F), and platelet (G) counts were also significantly increased in both heterozygous and homozygous Jak2V617F mice compared with controls. However, the WBC, neutrophil, and platelet counts were much greater in peripheral blood of Jak2V617F homozygous mice compared with heterozygous Jak2V617F mice. Blood counts at 4, 8, 12, 16, and 20 weeks after induction with pI:pC are shown. (n = 30 at all time points for V617F/+ control; n = 30 at all time points for heterozygous Jak2V617F; n = 10 at 4, 8, 12 weeks, n = 6 at 16 and 20 weeks for homozygous Jak2V617F mice). (H) Reticulocyte counts were markedly increased in the peripheral blood of homozygous Jak2V617F mice. (I) Serum Epo level was significantly reduced in both heterozygous (n = 9) and homozygous Jak2V617F (n = 6) mice compared with controls (n = 9). (J) BM cellularity (total BM cell count; was significantly reduced (12 to 16 weeks after induction) in mice expressing homozygous Jak2V617F. (K) Spleen weight/size was significantly increased in Jak2V617F heterozygous (n = 20) and homozygous (n = 9) mice compared with controls (n = 20; 12 to 16 weeks after induction). *Significance between control and heterozygous or between control and homozygous; **significance between control and homozygous as well as between heterozygous and homozygous Jak2V617F mice; P < .05 determined by unpaired, 2-tailed Student t test.