With additional research, Galon et al. further classified the tumor immune microenvironment (TIME) into four types, the first being hot tumors, in which the tumor tissue is infiltrated with a large number of T cells and cytotoxic T lymphocytes (CTL), with elevated expression of various immunosuppressive receptors on immune cells, such as programmed cell death protein 1 (PD-1), T-cell immunoglobulin (Ig) domain and mucin domain 3 (Tim-3), CTL-associated protein 4 (CTLA-4), and lymphocyte activation gene 3 (LAG-3). Here, PDCD1 is linked to neoplasm.