CD27 and cancer: Immune checkpoint genes play essential roles in cancer immunotherapy, which are categorized into co-stimulators (e.g., CD80, CD28, ICOSLG), co-inhibitors (e.g., PDCD1LG2, CD274, VTCN1, CD276), ligands (e.g., TIGIT, PDCD1, TNFSF9, CD70, CD40LG), and receptors (e.g., TNFRSF9, TNFRSF4, TNFRSF14, LAG3, ICOS, BTLA, CD27) [11].