Fig. 2. Generation of loss-of-function Blimp1 mutants by gene targeting. (A) Strategy used to engineer the prdm1^BEH allele. Correctly targeted clones were initially identified with the 5' external probe (red) and subsequently confirmed with the 3' external probe (blue). A, ApaI; E, EcoRI; M, MscI; N, NcoI; P, PstI; X, XbaI. (B) Southern blot analysis of EcoRI-digested genomic DNA from individual drug-resistant ES cell clones. The 5' external probe detects 8.2-kb wild-type (WT) and 7.9-kb targeted prdm1^BEH alleles. (C) A similar targeting strategy was used to generate the prdm1^BAH allele, containing an additional 1-kb deletion immediately 5' to exon 1 (see Materials and methods). The selection cassette is not shown.