A 14.5 kb genomic fragment was replaced with an IRES-betageo-pA cassette that was expected to join the endogenous exon 2 splice acceptor site. The deleted region included exons 2 through 6, which encode 95% of the extracellular domain and all of the transmembrane domain. RT-PCR and Western blot analyses indicated the absence of transcript and protein in homozygous mutant mice. (J:84392)