An optimized internal ribosome entry sequence (IRES) fused to cre recombinase followed by a neomycin resistance cassette flanked FRT sites was inserted after the ChAT stop codon in BAC (bMQ185k21) using homologous recombination in bacteria. A genomic ChAT fragment with 4kb homology arms on either side of the IRES-Cre insertion site was lifted out from the BAC using reverse recombineering techniques and transformed into ES cells. This allele was used to generate Chattm1(cre)Lowl.