ESR1 and breast cancer: Guideline-concordant treatment receipt was measured as the receipt of 1) any endocrine therapy among young adults with estrogen receptor–positive or progesterone receptor–positive breast cancer; 2) any chemotherapy or targeted therapy among young adults with estrogen receptor–negative and progesterone receptor–negative breast cancer; and 3) the first appropriate treatment among all young adults, defined as surgery, chemotherapy, or targeted therapy for stage I-III diagnoses and systemic therapy for stage IV diagnoses (7).