Human/Cynomolgus Monkey Midkine (MK) is a heparin-binding growth factor and cytokine that plays important roles in cell growth, survival, and tissue repair in humans (Homo sapiens) and cynomolgus monkeys (Macaca fascicularis). The midkine protein is identical at the amino acid level in these two species, reflecting strong evolutionary conservation and its essential biological functions in primates. Midkine is highly expressed during embryonic development and can also be induced in adult tissues in response to injury, inflammation, or physiological stress. Human and cynomolgus monkey midkine interact with several cell surface receptors, including receptor-type protein tyrosine phosphatase ζ (PTPRZ1), low-density lipoprotein receptor–related proteins (LRPs), and integrins, activating intracellular signaling pathways such as MAPK, PI3K–AKT, and NF-κB that regulate cell proliferation, migration, survival, and angiogenesis. Through these mechanisms, midkine contributes to processes including tissue regeneration, immune cell recruitment, neural development, and inflammatory responses. In cell culture, recombinant human/cynomolgus monkey midkine is commonly used to stimulate cell proliferation and migration and to study growth factor–mediated signaling pathways involved in development, inflammation, and tissue repair. Because cynomolgus monkeys are widely used as nonhuman primate research models, midkine is also studied in translational biomedical research to investigate mechanisms of development, tissue injury, and disease relevant to human health.