Protein that suppresses tumorigenesis. Tumor suppressors generally function as negative regulators of cell cycle progression or cell proliferation. They may act to enforce cell cycle arrest in response to specific signals, such as DNA damage, thereby allowing DNA repair to occur prior to DNA replication. Tumor suppressors that act in this way include TP53 and its downstream effector CDKN1A/p21. They may also act to induce programmed cell death (apoptosis) in response to specific developmental signals or under circumstances where DNA repair cannot be completed. Tumor suppressors that act in this way include PTEN and BAX.