There are likely to be up to 4500 isoforms of CYP in biota, including 57 in hum an tissues for isoform substrate specificities. These isoforms display differential inhibition by a wide range of biochemical drug-candi dates. Drugs are targeted to CYP19 (aromatase) and CYP17 (17 alpha-hydroxylase of sterols). The CYP19 isoform is rate limiting in oestrogen production in the body as it controls the biotransformation of testosterone to oestradiol by aromatisation of steroid ring A. A range of azoles is being developed for treatment of breast cancer by targeting the CYP19 (aromatase) isoform that catalyses the formation of an aromatic ring A as required for the conversion of testosterone to oestradiol. In addition, CYP17 (17 alpha-hydroxylase of pregnenolone and of progesterone) is targeted by a range of drugs for the treatment of prostate cancer. Inhibitor-CYP cot mparisons are recom nmended therefore for CYP19 and CYP17 in silico and real-lab so as to continue the development for instance of therapeutic azole inhibitors of CYP19 (aromatase) such as anastrozole, by recognition in silico of quantitative structure/activity relationships (QSARs) between such therapeutic drugs and CYP-utilising pathways. (C) 2007 Elsevier Inc. All rights reserved.