0.3 kb of the first coding exon was replaced with a neomycin selection cassette inserted by homologous recombination. Transcript containing the neo cassette was detected by RT-PCR analysis of mutant mice. Mutant protein potentially encoded by the detected transcript lacks the catalytic unit and is putatively nonfunctional. Inositol 1,4,5-triphosphate 3-kinase activity was attenuated in a dose dependent manner in mutant mice. (J:86257)