A systematic review and meta-analysis of 33 studies have demonstrated that high CD8+ T cells in tissue (i.e., intra-tumoral, stromal, or invasive marginal), but not circulating CD8+ T cells are predictive of treatment outcomes and better prognosis with immune checkpoint inhibitors across cancer types (e.g., NSCLC, melanoma, others) or treatment type (i.e., single-agent or combination therapy of immune checkpoint inhibitor) [49]. This evidence concerns the gene CD8A and melanoma.