Enhanced apoptosis of pro-B cells in the BM of STAT3KO mice. BM cells of control (▪) or STAT3KO (□) mice were incubated in the culture medium for 0 hour or 3 hours and were stained with anti-B220, anti-CD43, and annexin V, then FACS analysis was performed. Percentages of apoptotic pro-B (CD43⁺B220⁺annexin V⁺), pre-B/IMB/MB (CD43^–B220⁺ annexin V⁺), and non-B (B220^– annexin V⁺) were plotted (control mice, n = 8; STAT3KO mice, n = 5). Apoptosis results are presented as mean ± SE of replicate samples.