Human liver appears to contain at least 15 cytochrome P450 isoforms, falling into the three related families CYP1, CYP2, CYP3. The substrate specificities of this highly polymorphic group of enzymes are different, but overlapping. Birkett et al. (1993) state that even a single amino acid substitution may change substrate specificity. Wrighton et al. (1993) provide information on which subtypes are believed to oxidize particular drugs.