Staining of fetal BM mononuclear cells with fluorochrome-labeled monoclonal antibodies and sort criteria and purity of pro–B cells and sIgM⁺ B cells. Pro–B cells were defined by forward scatter (fsc) and sidescatter (ssc) properties (A) and the simultaneous expression of CD34 and CD19 (B). Mature B cells were defined by fsc and ssc properties (A) and the coexpression of CD19 and surface IgM (D). The purity of sorted pro–B (C) and B (E) populations was determined by rerun analysis on a FACSscan.