Surface FACS analysis of VpreB and μ chains on human proB-cell (RS4.11, JEA2, REH), preB-cell (NALM6, LAZ 221), and B-cell (NAMALWA) lines, using the anti-VpreB (4G7 and 10G5) and anti-μ MoAbs. The 4G7 MoAb is PE-labeled, whereas the 10G5 is indirectly shown by a PE-conjugated goat antimouse IgG+IgM (H+L). Isotype-matched MoAbs served as negative controls. When indicated (line 3), JEA2 cells were cultured with 20 ng/mL of IL-7 for 4 days. In that case, the 4G7 anti-VpreB labeling is completely inhibitable by 2 μg/mL of the COOH terminal VpreB peptide (boxed inset).