After excluding 724 patients with incomplete staging data (n = 698) or carcinoma-in-situ / pre-invasive disease (stage 0) breast cancer (n = 26), 13,100 patients were eligible for inclusion in the analytic cohort (Fig. 2); n = 265 patients with more than one HER2 result during the study period were retained in the analytic cohort. Here, ERBB2 is linked to breast cancer.