Cynomolgus Monkey Fas Ligand (FasL, CD95L, gene FASLG) is a type II transmembrane protein belonging to the tumor necrosis factor (TNF) superfamily, which includes related ligands such as TNF-α, TRAIL (TNFSF10), and CD40L (CD154) that regulate immune signaling, inflammation, and apoptosis. In cynomolgus monkeys (Macaca fascicularis), Fas ligand is primarily expressed on activated T lymphocytes, cytotoxic T cells, and natural killer (NK) cells, where it interacts with its receptor Fas (CD95) on target cells. Binding of FasL to Fas activates the extrinsic apoptotic pathway, triggering caspase-dependent signaling cascades that induce programmed cell death. In cynomolgus monkey immunity, the Fas–FasL system plays an important role in cytotoxic T-cell–mediated elimination of infected or abnormal cells and in maintaining immune homeostasis by removing activated or autoreactive lymphocytes. Because cynomolgus monkeys share close genetic and immunological similarities with humans, they are widely used as preclinical animal models for immunology, infectious disease, and transplantation research. In these models, Fas ligand signaling has been studied in nonhuman primate models of transplantation tolerance and immune regulation, where Fas-mediated apoptosis contributes to deletion of alloreactive T cells and modulation of graft rejection responses, providing insights relevant to human immune tolerance and transplant immunology.