Previous epidemiological studies, including the Nurses’ Health Study (Michels et al., 2006b; Hurley et al., 2011), have reported stronger positive associations between birth weight and the development of breast cancer in estrogen receptor-positive (ER+) and/or progesterone receptor-positive (PR+) tumors compared to estrogen-independent tumors. This evidence concerns the gene ESR1 and breast carcinoma.