ESR1 and breast cancer: For example, 21-gene recurrence score (RS), a score based on gene expression signatures indicating the risk of distant metastasis, has been incorporated into routine clinical care in the US.4,5 Although low SES and income levels are associated with a higher incidence of triple-negative breast cancer,6,7,8 the association between low income and RS among patients with estrogen receptor (ER)-positive breast cancer, the most common subtype, remains unclear.