Gender affirming treatment in transgender women is based on a combination of antiandrogens and estrogens, with the latter maintained over the long term, allowing the persistence of female secondary sexual characteristics. However, we must consider the possibility of developing estrogen-dependent breast cancer in transgender women undergoing such treatment.

The incidence of breast cancer in cisgender women and men is 12% and 0.1% respectively.1 In transgender women an incidence of 4.1 per 100,000 has been estimated,2 which would increase the risk by 46% in relation to cisgender men but decrease it by 70% in relation to cisgender women.

It is known that certain gene mutations such as BRCA1 imply an increased risk of breast cancer, but at present the risk in transgender women with BRCA1 treated with estrogens is not established.3

We present the case of a transgender woman with a family history of breast cancer and BRCA1 mutation and we will review and discuss the published literature.