A 600 bp fragment encompassing exon 3 and surrounding intronic sequence was replaced with a neomycin selection cassette inserted by homologous recombination. The deleted region encoded extracellular loop E, which contains the conserved Asn-Pro-Ala motif and the merucury-sensitive cysteine at position 182. Neither transcript nor protein was by analyses of salivary glands obtained from homozygous mutant mice. (J:82588)