Cytochrome P450 proteins are mainly drug-metabolising enzymes but are also important for metabolising some endogenously derived compounds such as cholesterol, prostacyclin, thromboxane A2, vitamins A and D etc. The P450 proteins are categorised into families and subfamilies. In humans there are 18 families and 43 subfamilies. Most drugs are metabolised by three families: CYP1, CYP2 and CYP3. The subfamily CYP3A is the most important and abundant protein, known to metabolise at least 120 drugs.