The CYP3A genes (CYP3A5, CYP3A7 and CYP3A4) encode enzymes that metabolise a diverse range of substrates;11 in addition to a role in the oxidative metabolism of hormones, CYP3A enzymes metabolise ~50% of all clinically used drugs, including many of the agents used in treating cancer.12 CYP3A4, the major isoform in adults, is predominantly expressed in the liver, where it is the most abundant P450, accounting for 30% of total CYP450 protein. The gene discussed is CYP3A4; the disease is cancer.