Loss of Mef2C in hematopoietic cells results in impaired B-cell homeostasis similar to premature B-cell aging. (A) FACS analyses of frequencies of B-cell subsets in bone marrow of VavCre⁺Mef2C^fl/fl and age-matched littermate control mice, for expression of B220, IgM, CD43, and CD24: representative examples of (i) young (1 month) and (ii) older (15 months) mice. Frequency of mature B220⁺IgM⁺ B cells (Fr. E-F) was not significantly altered regardless of age. Subgating on IgM⁻ cells visualizes pre-B (Fr. D) and pro-B (Fr. B-C) populations. Strikingly, older, but not young, mice exhibited severe reductions of pre-B cells. Subgating on B220⁺CD43⁺ cells visualizes pre-pro-B (Fr. A) and pro-B cells. Of note, a slight reduction in the frequency of pre-pro-B cells was noted in both young and older mice. (B) Summary of all mice analyzed. Circles indicate analysis of individual young (1-2 months) and older (9-10 and 14-15 months) mice, and values represent frequency in whole BM.