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 cholecystokinin locus (Cck). This construct was electroporated into C57BL/6;129 embryonic stem (ES) cells. Correctly targeted ES cells were injected into recipient blastocysts. Chimeric males were bred with Actin-FLPe females (on a C57BL/6 congenic background) to achieve germline transmission and to remove the neo selection cassette. The resulting pups harboring the cre targeted mutation were selected. These Cck-IRES-Cre mice were subsequently bred together for several generations (and the FLPe transgene was removed).