Cytochrome P-450 (CYP) isoenzymes are grouped into families designated by numerals (1, 2, 3....; each having several sub-families designated by capita: letters (A, B, C. .... ), while individual isoenzymes are again allocated numerals (1, 2, 3 .... ). In human beings, only a few, members of three isoenzyme families (CYP 1, 2 and 3 carry out the metabolism of most of the drugs, The CYP isoenzyme: CYP3A4/5 Carryout biotransformation of largest number: (nearly 50%) of drugs. 20-30% of metabolism is done by the next common cyp2d6 In addition to liver, these isoforms are expressed in the intestine (responsible for the first-pass metabolism at this site) and kidney as well. Ref: KD Tripathi pharmacology 7th edition (page.no: 24)