No other immunotherapy achieves the same degree of tumour cell killing,37 and anti-PD-1 agents in particular have revolutionised the treatment of metastatic melanoma, with durable responses occurring in more than 50% of patients surviving for the past 4 years.38 Between March 2011 and August 2018, six checkpoint inhibitors targeting the PD-1 pathway were approved for the treatment of 14 indications in the United States (three PD-1 inhibitors—pembrolizumab, nivolumab and cemiplimab, and three PD-L1 inhibitors—atezolizumab, avelumab and durvalumab). Here, PDCD1 is linked to metastatic melanoma.