In addition, we do not have prospective data on CYP3A7*1C carrier status, hormone levels and breast cancer risk in large numbers of women, and we cannot compare breast cancer risk in women with low levels of the parent oestrogen E1 according to their CYP3A7*1C status (and hence 2-hydroxylation:16α-hydroxylation ratio) directly. This evidence concerns the gene CYP3A7 and breast cancer.