A targeting vector was designed to insert an internal ribosomal entry site, nuclear-localized Cre recombinase gene (NLS-Cre), and polyA signal (IRES-Cre-pA), all followed by an FRT-flanked PGK-Neo cassette into the 3' untranslated region of the developing brain homeobox 1 (Dbx1) locus. This construct was electroporated into C57BL/6-derived Bruce4 embryonic stem (ES) cells. Chimeric mice were bred to albino B6(Cg)-Tyrc-2J/J mice. Next, mutant mice were bred with FLPe-expressing mice on a C57BL/6 congenic background to remove the FRT-flanked PGK-Neo cassette. The resulting Dbx1-IRES-Cre mutant mice were then bred to C57BL/6J inbred mice for many generations (selecting away from the FLPe transgene).