The endogenous locus was replaced with a single loxP site after the transient expression of cre recombinase in ES cells deleted sequence encompassing (from 5' to 3') the 3' end of a nonfunctional retrotransposon of the intracisternalA particle (IAP), the promoter, and exons 1 through 3. Neither transcript nor protein was detected in homozygous mutant mice by Northern and Western blot analyses.