A 3' portion of exon 3 was replaced with lacZ and neo, both simaltaneously inserted via homologous recombination of a targeting vector. The expression pattern of lacZ recapitulated that of the endogenous gene. Transcript was undetected by RT-PCR analysis of mRNA obtained from homozygous mutant kidneys. Immunostaining showed an absence of protein in sensory hair cells of homozygous mutant mice. (J:85071)