The authors describe a novel 30-kDa secretory protein, Acrp30 (adipocyte complement-related protein of 30 kDa), which is exclusively produced in adipocytes and whose mRNA expression is induced over 100-fold during adipocyte differentiation. Acrp30 shares structural similarities with complement factor C1q and a hibernation-specific protein from Siberian chipmunks, forming large homo-oligomers with post-translational modifications. Secretion of Acrp30 is enhanced by insulin, and it is an abundant serum protein. The study suggests that Acrp30 may play a role in energy homeostasis, possibly through its interaction with insulin-regulated pathways in adipocytes. The experiments also confirm the existence of an insulin-regulated secretory pathway in adipocytes.The authors describe a novel 30-kDa secretory protein, Acrp30 (adipocyte complement-related protein of 30 kDa), which is exclusively produced in adipocytes and whose mRNA expression is induced over 100-fold during adipocyte differentiation. Acrp30 shares structural similarities with complement factor C1q and a hibernation-specific protein from Siberian chipmunks, forming large homo-oligomers with post-translational modifications. Secretion of Acrp30 is enhanced by insulin, and it is an abundant serum protein. The study suggests that Acrp30 may play a role in energy homeostasis, possibly through its interaction with insulin-regulated pathways in adipocytes. The experiments also confirm the existence of an insulin-regulated secretory pathway in adipocytes.