Transgenic expression of the γ2a heavy chain gene. (A) T cell—depleted splenocytes from various transgenic mice were cultured in activators and IFN-γ as indicated at the top of the figure. Germline transcripts from the endogenous and transgenes were distinguished by a migration polymorphism, which is due in part to a 4-bp insertion in transgenic Iγ2a. The 820 CD40L + IFN-γ sample was run on the same gel as the other seven samples on the left. However, because of the increased expression of all germline transcripts in this sample, the radioactivity was very high. Therefore, a different exposure of this lane is shown. (B) Post-switch transcripts. In this Figure, and in Fig. 4, Fig. 5, and Fig. S4, intact and Δ cDNA samples were first equalized for the expression of IμCH transcripts, and then the same concentrations of cDNA were tested for VDJCH expression. The expression patterns of germline and post-switch transcripts expressed by B cells from lines 231Δ and 231 were identical to those of lines 336, 336Δ, 234, and 234Δ (not depicted). (C) HPRT mRNA expression. Two concentrations of cDNA were tested by RT-PCR, representing one-fifth and one-twenty-fifth of the amount of cDNA from the indicated cultures tested for post-switch transcripts in B. (D) µ-γ2a CSR of the endogenous and transgenes was tested by DC-PCR. In the schematic, the gray portion depicts sequences 5′ of Sµ and the black portion depicts sequences 3′ of Sγ2a. Dde1 sites (arrows in schematic) differentiate DC-PCR products from the endogenous genes and transgene. A 4-bp insertion in the transgene (black triangle) also differentiates one transgenic fragment from its endogenous counterpart. Each experiment shown was repeated in one or two additional experiments. Black lines between lanes indicate that the order of the lanes has been rearranged from the original electronic file, or that the lanes are derived from an independent experiment.