The inhibitory effect of enterolactone, a metabolite and marker of dietary lignans, on human aromatase by mammalian lignans and isoflavonoid phyto-oestrogens has been shown in vitro (Adlercreutz et al. 1993; Adlercreutz 1995), though the protective effects against breast cancer are only slight. This evidence concerns the gene CYP19A1 and breast cancer.