Cynomolgus Monkey (Macaca fascicularis) Leukemia Inhibitory Factor (LIF) is a multifunctional cytokine belonging to the IL-6 family—which includes IL-6, IL-11, oncostatin M (OSM), ciliary neurotrophic factor (CNTF), cardiotrophin-1 (CT-1), cardiotrophin-like cytokine factor 1 (CLCF1), and IL-27—and signals through receptor complexes containing the shared gp130 subunit. Highly homologous to human LIF, cynomolgus LIF binds to the LIF receptor (LIFR) in association with gp130, activating JAK/STAT3, MAPK, and PI3K signaling pathways that regulate cell survival, differentiation, immune modulation, and tissue repair. In nonhuman primates, LIF plays important roles in reproductive biology, including endometrial receptivity and embryo implantation, as well as in hematopoiesis, neural cell maintenance, and stem cell regulation. LIF-mediated STAT3 signaling is also involved in modulation of inflammatory responses and tissue remodeling during infectious disease models (e.g., dengue, influenza, SARS-CoV-2) and in studies of fibrosis and cancer biology. Dysregulated LIF expression may contribute to tumor progression or fibrotic pathology, making it relevant in oncology and safety pharmacology research. Because cynomolgus monkeys are widely used in preclinical evaluation of biologics and immune-modulating therapies, characterization of LIF supports translational investigations into IL-6 family signaling, STAT3-targeted therapeutics, reproductive medicine, and inflammatory disease mechanisms relevant to human health.