B progenitors from HB-LTC express mRNA for TdT, Rag, VpreB, μ, and light chain. RT-PCR was performed using the primer pairs as described in Materials and Methods with (lane 1) RNA from nonadherent cells of 8-week-old HB-LTC; (lane 2) RNA from S17 cells grown in the same conditions without addition of the CD34⁺ progenitors; (lane 3) no added RNA; (N) RNA from the human pre-B–cell line, Nalm-6; or (R) RNA from the human IgM⁺ immature B-cell line, Ramos. Lane M contains DNA size standards. Results are representative of at least three independent experiments using unrelated HB-LTCs.