Exon 2 of the endogenous locus was replaced by a cDNA consisting of exons 2-16 flanked by loxP sites, followed by an internal ribosomal entry site (IRES) to enable expression of beta-galactosidase from the inserted bacterial lacZ gene. Cre mediated recombination in the germline resulted in the deletion of the loxP-flanked sequences and expression of only the bacterial lacZ gene.