Exon 1, which encodes most of the protein and the entire CRD, was replaced by a LacZ/MC1-neo cassette. PCR analysis demonstrated that homozygous mutants contained the LacZ gene. Northern blot of kidneys indicated no presence of transcript for the CRD domain of the endogenous gene. RT-PCR analysis of calvaria showed lack of expression of both the CRD (the 5' region of the transcript) and the netrin domain (the 3' region) in mutants. (J:89547)