ID1 and neoplasm: Id1 absence in the tumor microenvironment in Id1-deficient mice significantly increased the tumor infiltration of CD3+ T cells, and more importantly, effector CD8+ T cells, as compared to Id1-expressing mice (CD3+ T cells p > 0.0001; CD8+ T cells p > 0.0001) (Figures 4A, B; Table 3).