A lacZ gene and neomycin resistance cassette were inserted into exon 2, just before the translation initiation codon. Two thirds of exon 2 and a part of intron 2 were deleted. This allele expresses beta-galactosidase under the control of the endogenous promoter. Northern analysis demonstrated that homozygous mice did not express wild-type mRNA. Immunohistochemistry on thymus, lung and cerebellum failed to detect wild-type protein in homozygous mice.