PCR analysis of completed VDJ rearrangement of the IgH locus in sorted early (Fr B) and late (Fr C) pro–B cells from 4-week-old Eμ-ret (Tg⁺) and transgene-negative (Tg⁻) mice. A 560-bp region upstream of the most V proximal D element, DFL16.1, that is lost on V to DJ rearrangement was amplified. Autoradiography was used to compare the signal intensity of the 5′ DFL16.1 amplification after standardization with an α-actin signal. Fr B cells from the Eμ-ret mice average 98% of the 5′ DFL16.1 signal intensity generated by Fr B cells from the transgene negative mice, whereas the Fr C cells average 43% of the signal intensity generated by the Fr C cells from the transgene-negative mice. Shown are 30-minute exposures of 26 cycles of amplification from the DNA of approximately 2 × 10³ cells.