The targeting vector contained, from 5' to 3', a partial Oxtr sequence spanning intron 1 up to but not including the endogenous stop codon in exon 2 (with a frt3 site in intron 1), a viral 2A oligopeptide sequence (T2A; mediates ribosomal skipping), a Cre recombinase gene, a bovine growth hormone polyA sequence, an AttB site, a PGK/gb2 promoter-Neomycin resistance gene-PGK polyA cassette, a frt5 site, an mRNA splice acceptor sequence, the 3' portion of the hygromycin gene (Hygro2) with SV40 polyA signal, and an AttP site. PhiC31-mediated recombination removed the AttB/AttP-flanked sequences (PGK/gb2-Neo-pA::frt5::RNA splice acceptor::3'hygro-pA) and replace it with the recombined AttB/AttP site (AttL).