Fas, also known as APO-1, CD95, or TNFRSF6, is a cell surface receptor that transduces apoptosis signals. It is one of two apoptosis pathways, the other being the mitochondrial pathway. Once bound to ligands, Fas forms a death-inducing signaling complex. Membrane-anchored Fas ligand trimers on adjacent cell surfaces lead to Fas hypomerization. Recent studies in mouse models have shown that Fas promotes tumor growth, but the human cancer genomics database suggests that Fas acts as a tumor suppressor. In mouse models of colon cancer induced by AOM-DSS and sarcoma induced by MCA, results showed that Fas was a tumor suppressor. In addition, Fas can also mediate the anti-tumor cytotoxicity of tumor-specific cytotoxic T cells. Studies have reported that serum Fas levels are elevated in patients with cancer and autoimmune diseases, and the former may originate from the tumor cells themselves.