PDCD1 and neoplasm: Among the identified biomarkers, circulating T cells are one of the most promising for three reasons: 1) It has been historically accepted that effective anti-PD-(L)1 therapy relies on the reactivation of pre-existing T cells of the tumor, which exhibit high programmed cell death protein 1 (PD-1) expression levels; however, recent data suggest that effective anti-PD-(L)1 therapy relies on the continuous recruitment of new T cells from circulation (5).