Cytotoxic T lymphocytes (CD8+ T cells) can be activated by tumor antigens to effectively kill cancerous cells as long as their responses are not constrained by negative regulators (e.g., cytotoxic T lymphocyte-associated protein 4 [CTLA-4] and programmed death [PD]-1) that serve as checkpoints to control immune reactions and either limit or prevent tissue damage caused by an overactive CD8+ T cell response. This evidence concerns the gene CTLA4 and neoplasm.