Biomarkers for response to immune checkpoint inhibitor therapy (ICI), such as the ratio of serum kynurenine/tryptophan, have shown promise in detecting negative outcomes for treatment in advanced melanoma and renal cell carcinoma patients treated with nivolumab, an antibody against programed cell death protein 1 (PD1) (Li H. et al., 2019). Here, PDCD1 is linked to melanoma.