CTLA4 and breast cancer: Nevertheless, mammary tumors have been classically considered to have low immunogenicity due to the heterogeneous expression of antigens in the primary tumor or metastases, modifications in the antigenic profile during progression, low levels of major histocompatibility complex (MHC) expression, release of suppressor cytokines, and expression of T-lymphocyte activation suppressor molecules, such as cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) and programmed cell death protein 1 (PD-1) [23].