The targeting vector contained, from 5' to 3', a partial Chrna6 sequence within exon 6 up to and including the endogenous stop codon, an internal ribosome entry site 2 sequence (IRES2; allows translation initiation in the middle of an mRNA 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 woodchuck hepatitis virus post-transcriptional regulatory element (WPRE; to enhance the mRNA transcript stability), a bovine growth hormone polyA sequence, an AttB site, a PGK/gb2 promoter-Neomycin resistance gene-PGK polyA cassette and an AttP site. Chimeric mice were bred with C57BL/6J mice. (J:101977)