ERBB2 and breast cancer: Compared to triple-negative and HER2-positive breast cancer, HR+/HER2− breast cancer displays a distinct immunosuppressive microenvironment, characterized by lower infiltration of effector immune cells such as CD8+ T cells and natural killer (NK) cells, and a higher abundance of immunosuppressive populations, including regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs) (12).