For instance, metformin can exert an anti-melanoma effect (Li et al., 2018) and promote combination treatment efficacy with anti-PD-1 and anti-cytotoxic T-lymphocyte-associated protein-4 (CTLA4) agents (Afzal et al., 2018; Cha et al., 2018). This evidence concerns the gene PDCD1 and melanoma.