ESR1 and breast cancer: Most of these cases (N = 29) had early-onset breast cancer (before age 45), invasive ductal tumors (N = 26), ER-positive staining (N = 16 of 25 invasive, 3 of 3 for in situ), and high proliferation as indicated by high Ki-67 staining (N = 16 of 19 invasive, 1 of 3 in situ).