Jak2V617F increases the number of erythroid colonies (BFU-E and CFU-E) and total numbers of colonies in a BM transplant model. (A) CFU assays in methylcellulose show an approximately 2-fold increase in the total number of colonies in BM (P < .01) and an approximately 5-fold increase in the spleens of Jak2V617F-compared with Jak2wt-expressing mice (P < .01). Distributions of all types of colonies were comparable with the exception of Jak2wt spleens, in which no GEMMs were detectable (n = 5 Jak2V617F BM; n = 4 Jak2wt BM; n = 8 Jak2V617F spleen; n = 4 Jak2wt spleen). (B) Total numbers of BFU-Es were increased approximately 4-fold in Jak2V617F BM compared with Jak2wt BM mice (P < .01; n = 5 Jak2V617F BM; n = 4 Jak2wt BM, in duplicate) and approximately 9-fold increased in spleens of Jak2V617F compared with Jak2wt mice (P < .01; n = 8 Jak2V617F BM; n = 4 Jak2wt spleens, in duplicate). (C) Growth of CFU-Es in the absence of cytokines and hypersensitivity of CFU-E colonies to EPO in Jak2V617F spleens (P < .01) but not in Jak2wt spleens.