PDCD1 and metastatic malignant neoplasm: Additionally, in the second line or above treatment for patients with various metastatic cancers, particularly for patients with PD-L1 negative or microsatellite stability (MSS)/mismatch repair deficiency (dMMR) or low tumor mutation burden (TMB), the efficacy of single-agent PD-1/PD-L1 inhibitors were even much lower (2).