ESR1 and neoplasm: Younger age (P < 0.0001), being diagnosed after 2007 (P = 0.004), ER or PR positivity vs. ER and PR positivity (OR 0.58 (95% CI 0.47 to 0.72); P < 0.001), and poorly differentiated or undifferentiated tumor grade vs. well-differentiated or moderately differentiated grade (OR 0.68 (95% CI 0.56 to 0.84); P < 0.001) were associated with lower odds of initiation, while being outside our reference site of California (P < 0.004) was independently associated with greater odds of initiation of endocrine therapy.