Generation of p105−/− mice. (A) Targeting strategy of the ankyrin-encoding region of the nfkb1 gene. The relevant structure of the murine nfkb1 gene is shown at the top. Targeting vector pPNT/IκBγ II and the targeted allele are shown at the middle and bottom, respectively. Closed boxes, exons of nfkb1 gene; open boxes, the SV40 p(A), phosphoglycerate kinase (PGK)-neo, and PGK-tk cassettes. The position of EcoRI and EcoRV sites are indicated by I and V, respectively. The diagnostic restriction fragments used for Southern blot analysis are indicated at the top (wild-type allele) and bottom (targeted allele). The DNA fragments used as 5′ external (EE), 5′ internal (XX), 3′ internal (E), and neo-specific (PP620) probes are indicated at the bottom. (B) Genotype analysis of pups generated from p105+/− intercrosses. Tail DNAs were digested with EcoRI, followed by Southern blot analysis using the 5′ internal probe XX. The wild-type allele is indicated by a 11.2-kb band, whereas the recombinant allele is represented by a 6.5-kb band. (C) Lack of p105 in homozygous mutants. Whole tissue extracts from thymus were subjected to Western blot analysis using a p50 antibody. Specific signals for the p50 isoform, p50, and p105 proteins are indicated by arrows.