An existing targeting vector was modified by inserting a codon-improved cre sequence in place of the EGFP sequence in the pFloxRI-C9a-EGFP construct. This construct was electroporated into PRX-B6N (Primogenix) for homologous recombination. Animals carrying the cre allele were crossed to PGK-cre mice to delete the floxed neomycin selection cassette. (J:199355)