Cre recombinase RNA was injected into zygotes heterozygous for Cyp1a1tm1Dwn. A region containing a floxed HPRT minigene and the entire coding region of Cyp1a1 was excised by cre recombinase. Northern and Western blot analyses of total RNA and protein extracts obtained from animals treated with TCDD (to enhance expression levels) showed an absence of endogenous transcript and normal protein in the livers of homozygous mutant mice.