In this work, the role of genetic as well as environmental factors in determining cytochrome P450 isozyme levels in man have been studied. Simple DNA based assays for the identification of individuals nulled at the CYP2D6 locus are described and have been applied to investigate whether this gene defect is associated with altered cancer susceptibility. In contrast to literature reports, in no cancer type were poor metabolizers underrepresented, indeed in several cancers the mutant allele frequency was increased. A model using human tumours grown as xenografts is described that should help elucidate the factors which regulate P450 levels in man.