An IRES-Cre-pA targeting cassette [33] was integrated into the 3′ UTR of exon 5, and ES cell recombinants were screened with a 5′ probe (oligos, 5′- GAGATGACCCAGCCAGGATGCCTC-3′ and 5′- CTGACCACTCTCGCTCCGGTGTCC-3′; genomic DNA, HindIII digest).