ERBB2 and breast cancer: In recent decades, there has been remarkable progress in the development of novel molecular targeted therapies (i.e., cyclin-dependent kinase 4 and 6 inhibitors, poly (ADP-ribose) polymerase inhibitors, and HER2 or trophoblast cell surface antigen-2 antibody-drug conjugates) and immunotherapies (i.e., programmed cell death-1 [PD-1] inhibitors) for patients with breast cancer who failed primary systemic treatment with endocrine therapy or chemotherapy (4).