Exons 7 through 9 were replaced with a single loxP site after homologous recombination and cre-mediate excision. The deleted region encoded the transmembrane, the soluble, and extracelluar membrane proximal regions. FACS analysis and assays of sera indicated a lack of an appreciable amount of both membrane-bound and soluble protein in homozygous mutant mice. (J:55526)