In vivo cre mediated excision of a floxed neo gene left the lacZ gene in place of the endogenous coding region. Transcript was undetected by Northern blot analysis of heart tissue isolated from newborn homozygous mutant mice. Western blot analysis showed a lack of protein in heart extracts from homozygous mutant embryos. The expression pattern of beta-galactosidase was similar to that of the endogenous gene. (J:79153)