The targeting vector contains, from 5' to 3', a partial Cartpt sequence spanning exon 1 through intron 2, an frt3 site within Cartpt intron 2, a partial Cartpt exon 3 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 sequence, the 3' portion of the hygromycin gene (Hygro2) with SV40 late polyA signal, and an AttP site.