CD4 and neoplasm: The surface expression of PD-1 and TIGIT was used as a marker of T cell exhaustion, and the frequency of CD4+ PD-1+ and CD4+TIGIT+ cells was markedly increased in both R270H and R172H mice, compared with the Control mice (Figs. 4G, H, S4J, K), suggesting that the CD4+ T lymphocytes in mutant p53 mice lost their anti-tumor activity.