Why is the defect of CSR in X4T B cells partial?
The CSR defect observed in the absence of X4 both in vitro and in vivo is partial. As shown in Fig. 3 C, X4T deletion efficiency is almost complete and so likely does not account for the partial nature of the CSR defect in X4T mice. Given the structure of the X4T transgene, with LoxP sites flanking the transcriptional unit upon virus integration (Fig. 7 A), the partial CSR defect observed in X4T B cells could reflect the persistence of X4T expression from the Cre-excised X4T cassette in mature B cells. To explore this possibility, we performed a Southern blot analysis using a probe specific for the third exon of the XRCC4 gene (Fig. 7 A, probe E), which is deleted in X4 KO mice (12). The 2.8-kb NdeI band revealed by probe E and specific for the X4 WT allele (Fig. 7 B, lane 13) was absent in all samples from mice deficient for endogenous X4, as expected (X4T and X4T mice; Fig. 7 B, lanes 1 to 12). In contrast, hybridization with probe E revealed the integrated X4T transgene as a 5.7-kb
