A Cre recombinase-estrogen receptor T2 (CreERT2) fusion-FRT-Neomycin-FRT-loxP cassette was introduced into the first coding ATG, replacing the last 4 bp of the exon 1. Homologous recombination was achieved in 129S6/SvEvTac-derived embryonic stem cells and resultant chimeric mice were crossed with Flp mice to excise the neomycin cassette,