Exons 2 through 5 were replaced with a neomycin selection cassette inserted by homologous recombination. The deleted region encoded amino acids 28 through 216, including serine 181 which has been implicated in the initial hydrolysis of phosphatidylcholine. Transcript was undetected by Northern blot analysis of homozygous mutant mice. Biochemical assays, including those of cholesterol esterification rate (CER), indicated a complete deficiency of alpha- and beta-lecithin cholesterol acyltransferase activity in homozygous mutant mice. A 70% reduction was observed in heterozygotes.