Cynomolgus Monkey (Macaca fascicularis) Interleukin-6 (IL-6) is a multifunctional pro-inflammatory cytokine and a central member of the IL-6 cytokine family—including IL-6, IL-11, IL-27, IL-31, leukemia inhibitory factor (LIF), oncostatin M (OSM), ciliary neurotrophic factor (CNTF), cardiotrophin-1 (CT-1), and cardiotrophin-like cytokine factor 1 (CLCF1)—which signal through receptor complexes containing the shared gp130 subunit to activate JAK/STAT and MAPK pathways. Due to the close genetic and immunological similarity between cynomolgus monkeys and humans, IL-6 in this species exhibits high structural and functional homology to human IL-6, making it highly relevant in translational research. In cynomolgus monkeys, IL-6 regulates acute-phase protein production (including C-reactive protein), B- and T-cell differentiation, hematopoiesis, fever, and systemic inflammatory responses during infection, tissue injury, and immune stimulation. Elevated IL-6 levels are observed in models of viral infections (such as dengue, influenza, and SARS-CoV-2), bacterial sepsis, autoimmune disease, and cytokine release syndromes, where IL-6 contributes to inflammation and disease severity. As a key biomarker of immune activation, cynomolgus IL-6 is widely used in preclinical safety, vaccine, and therapeutic studies, particularly in evaluating biologics targeting the IL-6/IL-6R pathway. Characterization of IL-6 and related family members in this species is critical for immunotoxicology, efficacy assessment, and translational modeling of human inflammatory, infectious, and autoimmune diseases.