In addition to diminishing efficacy, treatment discontinuation reduces the proportion of patients who receive subsequent lines of therapy (patient attrition rates), which among patients with HR-positive, HER2-negative breast cancer progressing from the first to the second line of therapy has been reported to be 15–33% in clinical trials and 18–51% in real-world evidence studies, primarily due to disease progression and death [33,39,40,41,42,43,44,45,46]. The gene discussed is ERBB2; the disease is breast carcinoma.