<qa>  What is the cytochrome P450 family located in?Enzymes of the cytochrome CYP P450 CYP2C19, CYP3A4, CYP3A5 family located in the liver and small intestine are responsible for the metabolism of PIs. The absence of expression of certain enzymes from this family was recently correlated with a genetic polymorphism, which may have a major role in variation of cytochrome P450-mediated drug metabolism. Results of these studies suggest significant differences in the distribution of the polymorphism associated alleles between ethnic groups, in particular between Caucasians and African Americans. Detection of cytochrome P450 variant alleles and more detailed data on their allelic frequency in various ethnic groups is critical to assess their impact on PK of antiretroviral agents, in particular PIs. 