This chapter discusses the genetic analysis of cytochrome P450 system. Molecular genetic analysis of cytochrome P450 enzymes represents an essential part of studies of the properties of this enzyme system. Such analysis allows the chromosomal assignment of P450 genes and the complexity and organization of specific gene clusters to be determined. This approach can also be used to determine the association of a specific P450 gene with a trait or phenotype, for example, cancer susceptibility, and also for the association of specific gene loci with the metabolism of specific drugs or hormones. Genetic analysis in mammals has been predominantly carried out in humans or the mouse. In the latter case, this is because of the availability of a wide range of inbred lines and the wealth of information already available on mouse genetics. Once a new gene or complementary DNA (cDNA) has been isolated one of the first steps in genetic analysis is to determine the chromosome assignment.