ESR1 and cancer: Breast cancer (BC) is one of the leading causes of cancer-related death for women in the US.1 While Black women have a slightly lower incidence rate of BC than White women, they are more frequently diagnosed with advanced disease and have a higher incidence of aggressive forms of BC, such as estrogen receptor (ER)–negative disease.2 These differences in BC stage at diagnosis and ER status contribute to racial disparities in BC outcomes.