Exon 1 was disrupted by the insertion of a cassette containing an IRES-tau-lacZ gene, a floxed neomycin gene, and a SV40 polyadenylation signal. The absence of encoded protein in the of homozygous mutant embryos was determined using antisera. Staining of heterozygotes showed the expression pattern of beta-galactosidase to be similar to that of the endogenous gene.