Differentiation patterns of sorted early (Fr B) and late (Fr C) pro–B cells from 4-week-old Eμ-ret (Tg⁺) and transgene-negative (Tg⁻) mice. A total of 2 to 4 × 10⁴ cells were cultured in IL-7 (100 U/mL) for 4 days and analyzed by flow cytometry for cell surface expression of CD43 and IgM. Mean percentages are given for duplicate cultures. Cells retaining CD43 expression correspond phenotypically to either the pro–B-cell or early pre–B-cell stage, whereas the cells losing CD43 expression correspond to the late pre–B (sIgM⁻) or B-cell stage (sIgM⁺). Fr C late pro–B cells from Eμ-ret mice retain a higher percentage of cells with CD43 expression.