As a consequence, immune-checkpoint proteins are frequently deregulated by tumors.[76] Cytotoxic T-lymphocyte antigen 4 (CTLA-4), which is a negative regulator of T cell activation, and PD-1 (programmed cell death protein 1), which is an immune checkpoint that limits the activity of T cells in peripheral tissues, are immune checkpoint proteins that are currently being targeted in a variety of solid tumors, such as melanoma, lung, renal and also breast cancer. Here, PDCD1 is linked to breast cancer.