PDCD1 and mucosal melanoma: Additionally, in a subgroup of patients with oral mucosal melanoma, retrospective data suggests significant improvement in two-year overall survival and progression-free survival with adjuvant anti-PD-1 combined with chemotherapy compared with chemotherapy alone, consistent with the higher tumour mutational burden and interferon expression seen in head and neck mucosal melanomas compared with other mucosal subgroups [41].