The targeting vector contains, from 5' to 3', a partial Npy sequence spanning exon 4 (including the endogenous stop codon), an internal ribosome entry site 2 sequence, a FlpO recombinase gene (a codon-optimized FLPe gene modified for translation in mammalian cells and higher recombination efficiency in cells and in mice), 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 polyA signal, and an AttP site. PhiC31-mediated recombination removed the AttB/AttP-flanked sequence (PGK-Neo-polyA::frt5::RNA splice acceptor::3'hygro-polyA) and replaced it with the recombined AttB/AttP site (AttL).