The targeting vector contains from 5' to 3', a partial Penk sequence spanning exon 1 to intron 1 (with an frt3 site in intron 1), a partial Penk exon 2 sequence up to and including the endogenous stop codon, an internal ribosome entry site 2 (IRES2) sequence (allows translation initiation in the middle of an mRNA sequence), a Cre recombinase gene, a bovine growth hormone polyA sequence, an AttB site, a PGK promoter-Neomycin resistance gene-PGK polyA cassette, an frt5 site, an mRNA splice acceptor, the 3' portion of the hygromycin gene (Hygro2) with SV40 late polyA signal, and an AttP site.