PDCD1 and melanoma: In melanoma patients, an increased TIGIT/CD226 ratio in tumor-residing Tregs (increased TIGIT expression and decreased expression of its competing costimulatory receptor CD226) is associated with highly suppressive Treg function and poor clinical outcomes upon ICI blockade [80], while circulating populations of PD-1+TIGIT+CD8+T-cells can predict the efficacy of anti-PD-1 therapy [81].