PDCD1 and neoplasm: Among the biomarkers of TSTs initially identified in different tumor types, the immunomodulatory suppressor receptor programmed cell death receptor 1(PDCD1), the chronic antigen activation marker CD39 (ENPD1), the tissue-resident marker CD103 (ITGAE) and the costimulatory receptor CD137 (4-1BB, TNFRSF9) were mainly targeted as objects of interest (16–19).