PDCD1 and neoplasm: Recently, CD8+ tumor-infiltrating lymphocytes (TIL) were identified as displaying high expression levels of exhaustion markers such as T cell immunoreceptor with Ig and ITIM domains (TIGIT), CTLA-4, programed cell death 1 (PDCD1), hepatitis A virus cellular receptor 2 (HAVCR2), lymphocyte-activation gene 3 (LAG3), Layilin (LAYN), inducible T-cell costimulator (ICOS), eomesodermin (EOMES), and granzyme K (GZMK) [19, 20].