PDCD1 and melanoma: Early trials in the 1980s also showed that delivering the T cell stimulatory cytokine IL-2 can lead to tumour regression in melanoma and renal cell carcinoma (RCC) [3], while landmark clinical trials in the early 2010s established that monoclonal antibodies (mAbs) blocking the T cell inhibitory checkpoint molecules CTLA-4 and PD-1 led to improved survival in several cancer types [4,5].