PDCD1 and metastatic melanoma: P21 (RAC1) Activated Kinase 4 (PAK4) was consistently enriched in tumor biopsies with low T-cell (q-value = 2.75e-07) and dendritic cell (q-value = 1.9e-05) infiltration and was validated using an independent cohort of 99 metastatic melanoma biopsies treated with anti-PD-1 published by Riaz et al. Cell 2017 (q-value= 1.59e-11).