λ5 and V_preB expression in CD19⁻ and CD19⁺ cells generated in response to FL + IL-7. mRNA expression of λ5, V_preB , and HPRT (control) was investigated by RT-PCR on CD19⁻ (−) and CD19⁺ (+) cells isolated by fluorescence-activated cell sorting (FACS) as described in the Materials and Methods. For all three genes, the PCR amplification was stopped at intervals of five cycles (starting at 15 cycles). The PCR products generated by using primers for λ5, V_preB , and HPRT, resulted in products of 371, 343, and 248 bp, respectively.