Inter-individual variability in drug metabolism results from the influence of a myriad of factors, such as concomitant drug therapy and genetic factors. Advances in recombinant DNA technology have enhanced our understanding of the extent of genetic variation in the cytochrome P450 (CYP) enzyme super family, thus clarifying the molecular basis of many clinically observed variations in drug response. This second article in the CYP series describes current understanding of genetic variability in the major drug metabolising CYP enzymes, nomenclature used to describe variant CYP genes, and the clinical significance of such variability. J Pharm Pract Res 2008; 38: 226-9.