Immunotherapy with monoclonal antibodies (mAbs) that block PD-1/PD-L1 interactions on immune cells has shown remarkable success in the treatment of melanoma (Hersey and Gowrishankar, 2015, Hodi et al., 2016, Ribas et al., 2016, Ugurel et al., 2016) and other malignancies (Gandini et al., 2016, Ansell et al., 2015). Here, PDCD1 is linked to melanoma.