A TGK-neomycin resistance cassette replaced sequences spanning exon 2 through exon 13. The cassette introduced a lacZ sequence in frame with the endogenous promoter. The authors predict that the targeted allele would not produce a functional protein, but would produce a beta-galactosidase fusion protein controlled by the endogenous promoter. Western analysis did not detect wild-type protein in lactating mammary glands of homozygous mutant mice.