Figure 2.

Tumor suppressor gene classes. Three well-defined tumor suppressor classes can be defined based on the primary function of the proteins they encode. Anti-oncogenes function by encoding proteins that antagonize oncogene activity, such as with CDK4 (CDK4) and cyclin D1 (CCND1) being inhibited by the protein products of cyclin-dependent kinase inhibitor 2A (CDKN2A) and retinoblastoma (RB1). DNA damage checkpoint genes respond to DNA damage and replication stress by inducing cell death or senescence via ataxia telangiectasia mutated (ATM) and TP53. Caretaker genes such as MLH1 and BRCA1 promote genomic stability.