This may reflect the fact that compared to CYP3A4, CYP3A7 is a poor metaboliser of tamoxifen,32 or that standard doses of tamoxifen achieve high levels of oestrogen receptor saturation.33 There was some evidence that breast cancer-specific survival was reduced in CYP3A7*1C carriers who were treated with a taxane, compared with non-carriers (P = 0.01); this may, however, be a chance finding given the number of comparisons that were tested. Here, CYP3A4 is linked to breast cancer.