A targeting vector was designed to insert an internal ribosome entry site (IRES), a Cre recombinase sequence, an SV40 polyA signal, and an frt-flanked neo cassette into the 3' untranslated region (~60 bp after the translational termination site) of the Calb2 (calbindin 2; also called calretinin or CR) gene. Mutant mice were bred with Actin-FLPe mice (on a C57BL/6 congenic background) to remove the neo selection cassette and the transgene was subsequently bred out of the background.