We identified 1 136 016 patients (mean [SD] age, 62.4 [13.1] years; 99.1 female; 78.6% non-Hispanic White), comprising 392 246 patients (34.5%) diagnosed with ERBB-negative and 743 770 (65.5%) with ERBB2-low breast cancer who met the criteria for study inclusion (Table 1 and eFigure 1 in Supplement 1). Here, ERBB2 is linked to breast cancer.