HHLA2 and pancreatic neuroendocrine tumor: These studies specifically examined HHLA2 expression across a range of endocrine cancer types: five studies focused on pancreatic cancer (PC), two on ovarian cancer (OC), and one study each on papillary thyroid carcinoma (PTC), medullary thyroid carcinoma (MTC) and gastrointestinal neuroendocrine tumors (GINETs) along with pancreatic neuroendocrine tumors (PNETs).