RT-PCR analysis of TdT expression in sorted early (B) and late (C) pro-B cells from neonatal Eμ-ret (Tg+) and transgene-negative (Tg−) mice. Shown are ethidium bromide staining patterns of 26 (β-actin) or 30 cycles (TdT) of amplification resolved on a 1.5% agarose gel.