Okadaic acid is a diarrhetic shellfish toxin initially produced by dinoflagellates and sea sponges; it exhibits neurotoxic and anticancer activities. Okadaic acid inhibits protein phosphatases 1 and 2A (PP1, PP2A). In animal models of Alzheimer’s disease, okadaic acid increases phosphorylation of tau protein. In adipocytes, this compound stimulates lipolysis by increasing phosphorylation of perilipin A and B. In T cell leukemia cells, okadaic acid induces G1 phase cell cycle arrest, decreases expression of cyclin D2, CDK4, CDK6, Bcl-2, Bcl-xl, and XIAP, increases expression of p21 and p27, activates caspases, and inhibits cell growth.