CSF1R is expressed on early B-cell progenitors in the fetal liver but not in adult bone marrow. (A) Mean (± standard error of the mean [SEM]) Csf1r expression levels measured by RT-qPCR in 20 E13.5 FL (n = 7 embryos) and 8- to 10-week-old adult BM (n = 6 mice) ProB (Lin^–B220⁺CD43⁺CD19⁺CD24⁺CD93⁺) cells (2-3 experiments). Results are presented relative to Hprt1 expression. **P < .01. (B) Representative FACS profiles of CSF1R expression on ProB at E13.5, E14.5, and on ProB and PreB (Lin^–B220⁺CD19⁺CD43^–IgM^– cells) at E17.5 FL and in 8- to 10-week-old adult BM (2-5 experiments). (C) Mean percentage (± SEM) of ProB cells expressing CSF1R at E13.5 (n = 26 embryos), E14.5 (n = 21 embryos), E17.5 (n = 12 embryos), and in 8- to 10-week-old adult BM (n = 17 mice). (D) Representative FACS profiles of CSF1R expression on HSCs (Lin^–KIT⁺SCA-1⁺CD48^–CD150⁺FLT3^–) and LMPPs (Lin^–KIT⁺SCA-1⁺FLT3^high) in E13.5 FL and 8- to 10-week-old adult BM (2-5 experiments). (B, D) Numbers represent mean percentage of CSF1R⁺ cells in the investigated populations. (E) Mean percentage (± SEM) of HSCs and LMPPs expressing CSF1R at E13.5 (n = 21 embryos), E14.5 (n = 13-30 embryos), and 8- to 10-week-old adult BM (n = 6-17 mice). (F) Gene expression analysis of single E13.5 FL CSF1R⁺ ProB and CSF1R⁺ FL LMPP cells expressing Csf1r mRNA (2-3 experiments). Results are presented relative to Hprt1 expression. Each column represents a single cell.