Homologous recombination introduced a floxed neo cassette preceding the exon encoding transmembrane span 1 and part of transmembrane span 2 and a third loxP site was introduced after the exon encoding the remainder of transmembrane span 2. The floxed regions were deleted via transient expression of cre recombinase in correctly targeted ES cells. RT-PCR analysis detected a truncated transcript in mutant animals. Western blot analysis did not detect protein product in various tissues from homozygous mutant mice.