Expression of human enzymes in bacterial cells has made possible the development of a new generation of mutagenicity assays. Metabolic activation takes place within the internal milieu of the target cell, so that no membrane barrier is interposed between the reactive metabolite and the DNA target. This technology has been extended to all major classes of enzymes of xenobiotic biotransformation, including P450. Expression of sequence variants of mutagen-bioactivation enzymes provides a new tool for studying their structure and function.