A targeting vector was designed to insert an internal ribosome entry site (IRES), a Cre recombinase sequence, a polyA sequence, and an frt-flanked neo cassette into the 3' untranslated region (after the translational termination site) of the corticotropin releasing hormone locus (Crh). The mice were bred to actin-Flpe animals to remove the neo cassette and the Flpe transgene was subsequently bred out of the line.