Programmed cell death protein 1 (PD-1) inhibitors are foundational therapeutics in the treatment of metastatic melanoma, and assessment of BRAF V600E/V600K ctDNA levels in patients receiving anti-PD-1 treatment has been shown to be an accurate predictive factor for tumor response, PFS, and OS (Lee et al., 2017). The gene discussed is PDCD1; the disease is metastatic melanoma.