Among young adults with stage I-III estrogen receptor–positive or progesterone receptor–positive breast cancer (n = 33 166), the percentage receiving endocrine therapy increased in expansion states (85.21% pre-expansion to 86.46% postexpansion) but decreased in nonexpansion states (84.32% to 82.79%), leading to a net increase of 2.42 percentage points (95% confidence interval [CI] = 0.56 to 4.28 percentage points) in adjusted difference-in-differences models (Table 1;  Supplementary Table 3, available online). Here, ESR1 is linked to breast carcinoma.