Similar associations between PD-L1 expression and tumor grade have been reported in metastatic gastroenteropancreatic neuroendocrine tumors (GEP-NET ref. Consistent with these findings, expression of PD-L1 was rare among archival tissue samples from low-grade NETs of the small intestine ( n = 64) and pancreas ( n = 31 ref. Across NET sites, PD-L1 expression was detected in 0% of grade 1, 78% of grade 2, and 100% of grade 3 tumors ( 16). For example, PD-L1 expression was reported in 59% of pulmonary NETs ( 14) and 54% of insulinoma-like pancreatic NETs (pNET ref. Several studies suggest that programmed death ligand 1 (PD-L1) expression plays a role in the development, progression, and prognosis of NETs, especially in high-grade tumors. In cancer, PD-1 promotes tumor escape from host immune responses ( 12, 13). Programmed death 1 (PD-1) is a T-cell coinhibitory receptor that regulates immune response by interacting with its ligands (PD-L) ( 11).