Analyzing pre and on-treatment peripheral blood samples of stage IV melanoma patients treated with pembrolizumab, Huang et al. [100] report the on- treatment fold change in Ki67+/PD-1+ CD8+ T cells by 6 weeks of first dose of anti-PD-1 administration to correlate with objective response rate, progression free survival and overall survival after adjusting for a measure of tumor burden at baseline. The gene discussed is PDCD1; the disease is melanoma.