Even though the expression of PD-1/PD-L1 or CTLA-4 is unknown in normal parathyroid tissue, PD-L1 expression was demonstrated by immunohistochemistry in 8 (30.8%) of 26 parathyroid carcinomas and in 18 (48.6%) of 37 parathyroid adenomas [26]. Here, PDCD1 is linked to parathyroid gland adenoma.