Homozygous JAK2V617F expression increases the number of adult erythroblasts with increased proliferation but does not reduce apoptosis; these changes are associated with expression of genes related to cell cycle and/or mitosis. (A) JAK2^R/R mice show a marked increase in CD71⁺Ter119⁺ erythroblasts. Flow cytometry was performed on BM and spleen mononuclear cells (MNCs) using Ter119 and CD71 antibodies. (B) JAK2V617F expression does not affect erythroblast apoptosis. Flow cytometry was performed using Annexin-V and 7-AAD on BM and spleen erythroblasts. Histograms show percentages of apoptotic cells in CD71⁺Ter119⁺ erythroblast populations from both BM and spleen. (C) Homozygous JAK2V617F expression increased erythroblast proliferation. Lineage-negative BM cells were cultured to assess erythroid differentiation in vitro. A 5-bromo-2′-deoxyuridine (BrdU) cell proliferation assay was performed after 4 days in culture on the gated erythroblast population as shown in the plots with the G0/G1, S, and G2/M phases indicated. Asterisks indicate significant differences by Student t test (**P < .01). Data are shown as mean ± SEM. (D) Gene set enrichment analysis (GSEA) showing enrichment for STAT5 and GATA1 target genes in JAK2^R/R samples relative to non-JAK2^R/R samples (pooled datasets from 3 biological replicates each of WT and JAK2V617F^R/+ littermates). (E) GSEA showing enrichment of gene sets associated with DNA-damaging agents and the p53 response in JAK2^R/R samples. (F) GSEA showing enrichment in the JAK2^R/R samples for gene sets related to regulation of the cell cycle, but no enrichment for genes associated with apoptosis.