CSR occurs in AID-transgenic/AID−/− mice. A, Germline transcripts (GLTs) were analyzed by semi-quantitative RT-PCR using cDNAs derived from splenic B cells from AID-transgenic/AID−/− and WT mice that were activated with LPS or LPS +IL4 for 48 hours. Gapdh RT-PCR products were harvested after 29 cycles (lanes 1, 4, 7,10), 27 cycles (lanes 2, 5, 8, 11), 25 cycles (lanes 3, 6, 9, 12). GLT and AID RT-PCR products were harvested after 33 cycles (lanes 1, 4, 7, 10), 31 cycles (lanes 2, 5, 8, 11) and 29 cycles (lanes 3, 6, 9, 12). AID originating from the endogenous (e) alone (Muramatsu et al., 2000) or from a combination of endogenous genes and transgenes (e+Tg) (see Materials and Methods) are shown. B, Post-switch transcripts (PSTs) were analyzed by semi-quantitative RT-PCR using cDNAs derived from splenic B cells from AID-transgenic/AID−/−, AID+/+ and AID−/− mice that were activated with LPS or LPS+IL4 for 5 days. Gapdh PCR products were harvested after 29 cycles (lanes 1, 4, 7), 27 cycles (lanes 2, 5, 8) and 25 cycles (lanes 3, 6, 9). PST PCR products were harvested after 33 cycles (lanes 1, 4, 7), 31 cycles (lanes 2, 5, 8), and 29 cycles (lanes 3, 6, 9).