An elevated BMI is associated primarily with postmenopausal, estrogen receptor (ER)-positive (ER+) breast cancer; however, in pre-menopausal women, obesity is associated with a lower risk of ER+ breast cancer and a higher risk of triple negative breast cancer (TNBC) [6,7,8,9,10,11]. Here, ESR1 is linked to obesity due to melanocortin 4 receptor deficiency.