A greater understanding of the molecular processes involved and a systematic study of factors such as, i) whether circulating and intratumoural oestrogen are reduced, ii) the patency of oestrogen signalling pathways, iii) the degree to which proliferation is suppressed, and iv) the involvement of other pathways signalling for growth and cell survival in individual tumours appear to be important and necessary steps by which to optimise treatment with aromatase inhibitors such as letrozole. Here, CYP19A1 is linked to neoplasm.