PDCD1 and metastatic melanoma: However, durable responses are only observed in 40–60% of patients with metastatic melanoma who receive anti-programmed cell death protein 1 (anti-PD-1) monotherapy or anti-PD-1/anti-cytotoxic T-lymphocyte-associated protein 4 (anti-CTLA-4) combination therapy, highlighting the need for highly specific biomarkers to identify ICI-resistant patients and prevent overtreatment6.