A 3.6 kb fragment was replaced with a beta-galactosidase gene followed by a floxed neo gene. The deleted region included a 3' portion of the exon encoding the first PDZ domain. No full length protein was detected by Western blot analysis of protein extracts obtained from homozygous mutant embryos. Beta-galactosidase staining indicated that the reporter gene was expressed via the endogenous promoter.