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The insertion of an hprt gene, with the opposite orientation of the endogenous gene, deleted a portion of exon 2 and all of exons 3 through 5. This deleted region encoded the conserved cysteine-containing fragment and the cytochrome P450 tridecapeptide. Northern blot analysis of total hepatic RNA obtained from beta-naphthoflavone-treated animals showed a lack of transcript in homozygous mutant mice. Failure of homozygous mutant mice to metabolize zoxazolamine, a muscle relaxant, indicated a lack of functional protein. (J:31730)