CTLA4 and neoplasm: Moreover, the proximity of CD8+ T cells to tumor cells does not guarantee a strong immune response, as they overexpress immune checkpoint genes (ICGs) such as programmed cell death protein 1 (PD-1; encoded by the Pdcd1 gene), lymphocyte activation gene 3 protein (LAG3), and cytotoxic T-lymphocyte-associated protein 4 (CTLA4).