CD27 is a transmembrane protein that belongs to the tumor necrosis factor receptor superfamily (TNFRSF) and is primarily expressed on T cells, B cells, and natural killer (NK) cells. It activates NF-κB and JNK signaling pathways by binding to its ligand, CD70, thereby promoting the activation, proliferation, and differentiation of T cells. CD27 plays a crucial role in immune regulation, acting as an important co-stimulatory receptor for T and B cells, and is involved in modulating immune responses and the formation of memory cells. Additionally, CD27 is being studied as a potential target for anti-tumor immunotherapy.