A natural bicyclic depsipeptide that selectively inhibits class I HDACs (IC50s = 53, 39, 53, and 26 nM, for HDAC1, 2, 3, and 8, respectively) over HDAC4, 6, 7, and 9 (IC50s = 470, 330, 3,200, and 12,000 nM, respectively); has anti-cancer activities, particularly against certain T cell lymphomas; increases mRNA expression, histone acetylation, and the levels of proteins involved in NMDA receptor function in the brain of Shank3-deficient mice; rescues autism-like deficits in Shank3-deficient mice for at least three weeks following a dose of 0.25 mg/kg per day for three days