PDCD1 and melanoma: Based on our previous experience in the field of melanoma markers- and therapeutic targets identification [23–32], in the present study we selected nine molecules referred to as immune checkpoint related (ICR) molecules (namely: inhibitory targets such as CTLA4, PD-1, PD-L1, LAG3, TIM3, and TIGIT, and co-stimulatory molecules such as OX40 (TNFRSF4), GITR (TNFRSF18), 4-1BB (TNFRSF9).