Effects of the Y201F mutation on hematopoietic progenitors expressing JAK2V617F. (A) Flow cytometric analysis demonstrates significant decrease in Ter119⁺CD71⁺ erythroid precursors and Gr-1⁺Mac-1⁺ myeloid population in the spleens of mice transplanted with JAK2V617F/Y201F-transduced BM compared with mice transplanted with JAK2V617F-transduced BM. Representative dot plots are presented. (B) Histogram shows the percentage of Gr-1⁺Mac-1⁺, Ter119⁺, Ter119⁺CD71⁺, and CD71⁺ populations in the spleens (n = 4 for mice expressing vector control or JAK2WT; n = 6 for mice expressing JAK2V617F; n = 7 for mice expressing JAK2V617F/Y201F). Noticeably, the Y201F mutation markedly inhibited the JAK2V617F-evoked increase in Gr-1⁺Mac-1⁺, Ter119⁺, Ter119⁺CD71⁺, and CD71⁺ populations, and there were no significant differences in those populations among mice transplanted with vector control, JAK2WT-, or JAK2V617F/Y201F-transduced BM. (C) Analysis of Epo-independent CFU-E colony formation in the spleen. A total of 1 × 10⁵ spleen cells were seeded in methylcellulose medium without any cytokine (Methocult M3234). CFU-E colonies were scored after 2 days. Data are mean ± SEM from 3 independent experiments. *P < .05 (1-way ANOVA). **P < .005 (1-way ANOVA).