Fetal origin of human B-cell development. Pre-pro-B cells (CD19⁺CD10⁻CD34⁺) originate from the fetal liver and are abundant in fetal bone marrow (BM; shown here, left). In adult bone marrow, pre-pro-B cells are exceedingly rare (right) and differ from their fetal counterparts functionally and transcriptionally. See Figure 2A in the article by O’Byrne et al that begins on page 1059.