ESR1 and breast cancer: Menopausal hormonal therapy (MHT) is associated with increased breast cancer risk.1,2 The magnitude of the association decreases with time since cessation and reaches a similar level as the general population after 5 years.1,3 MHT use is more consistently associated with breast cancer risk in leaner women.1,2,4,5,6,7 Associations have been shown to vary by formulation.2 Increased breast cancer risk with estrogen-progestin therapy (EPT) use has been found for both estrogen receptor (ER)-positive and ER-negative tumors,1,3,5 but with more substantial associations for the ER-positive subtype.