Exon 2, intron 2, exon 3, and part of intron 3 were replaced with an in-frame lacZ gene inserted at the initiation codon followed by a neomycin selection cassette. Northern and western blot analysis demonstrated that no detectable wild-type transcript or protein was expressed from this allele. Histochemical analysis demonstrated that beta-galactosidase was expressed from this allele under the control of the endogenous promoter.