Cynomolgus monkey IL-2 (Interleukin-2) is a central T cell–derived cytokine that regulates lymphocyte proliferation, differentiation, and immune homeostasis in Cynomolgus monkeys (Macaca fascicularis). The IL-2 protein is highly conserved among non-human primates and is functionally comparable to that of Rhesus monkey (Macaca mulatta) and humans, supporting its translational relevance in preclinical research. IL-2 is produced primarily by activated CD4⁺ and CD8⁺ T lymphocytes following antigen recognition and costimulatory signaling, and it functions through the IL-2 receptor complex (IL-2Rα/CD25, IL-2Rβ, and the common γ chain) to promote clonal expansion of antigen-specific T cells, enhance cytotoxic T lymphocyte and natural killer (NK) cell activity, and support development and maintenance of regulatory T cells (Tregs). In healthy Cynomolgus monkeys, basal IL-2 expression is typically low but increases during infection, vaccination, or immune stimulation. Cynomolgus monkeys serve as important preclinical models for human infectious diseases, autoimmune disorders, transplant biology, and immuno-oncology. In translational and safety pharmacology studies, IL-2 is used as a biomarker of T cell activation and cell-mediated immunity and is particularly relevant in evaluation of biologics, vaccines, and immunotherapies targeting T cell pathways.