In addition to defining TSTs using CD39+PD-1+ in human cancer tissues, researchers also focused on circulating CD39+PD-1+CD4+ T cells in patients with human papillomavirus (HPV)-induced tumors, a cell subpopulation enriched with activated HLA-DR+ and inducible T cell costimulator (ICOS)+ and proliferating KI67+ cells in the peripheral circulation, as well as a high proportion of HPV-specific T cells. This evidence concerns the gene PDCD1 and cancer.