5). Heterogeneity in patients' responses to tamoxifen among breast cancer patients is consistently observed across patient populations where administration of the same dose of this drug results in a range of outcomes which include adverse events or therapeutic failure (Refs 6, 7). The complex metabolism of tamoxifen is primarily catalysed by cytochrome P450 (CYP) enzymes, amongst which CYP2D6, CYP2C19, CYP3A4, CYP2B6 and CYP2C9 are presumed to be the most important isoenzymes (Ref. 8). Here, CYP2C19 is linked to breast carcinoma.