Results support the utilization of ctDNA as a potential predictive biomarker for immune therapy response.254 Another analysis of 18 MSS metastatic CRC patients has identified ctDNA as a predictive biomarker for the therapeutic efficacy of nivolumab immunotherapy.255 Meanwhile, undetectable ctDNA, combined with high TMB and a decrease of cell-free DNA can be used in predicting response to checkpoint inhibitors and overall survival in metastatic melanoma patients with anti-PD-1 and anti-CTLA-4 combined treatment.256. Here, PDCD1 is linked to metastatic melanoma.