FIGURE 5.

In situ deletion of hs4 in Blxhs4 transformants. A, Genomic Southern blotting of transformants carrying the Blxhs4 transgene with (+) and without (−) hs4. The BAC copy numbers for these clones are three (clone 6.1), six (clone 7.1), three (16.1), one (137.1), and one (169.1). The probe (hs3b) is indicated in the maps in A. This probe also detects a smaller HindIII fragment surrounding hs3a, but the blot was cut before hybridization to remove this fragment. Diagrams of the endogenous IgH locus and the Blxhs4 transgene before and after loxP-mediated deletion of hs4 are shown. Relevant HindIII (H) sites are shown; other symbols are as described for Fig. 2. Quantification of the blots suggests that hs4 deletion was accompanied by loss of one BAC copy in clones 7.1 and 16.1. B, Northern blot of RNA from five independent Blxhs4 transformants (6.1, 7.1, etc.) before and after hs4 deletion. The probes used were specific for α and γ2a mRNA. The parental cell line 9921 (expressing Igγ2a only) and the α-expressing J558 cell line are included as controls. C, Bar graph of data obtained from three independent experiments (Northern blots) measuring α-mRNA expression before (□) and after (■) hs4 deletion. The data are plotted for each transformant relative to initial expression level (before hs4 deletion) of 100%.