Tetrahydrocannabinol (THC), or more precisely its main isomer (−)-trans-Δ9-tetrahydrocannabinol ((6aR,10aR)-delta-9-tetrahydrocannabinol), is the principal psychoactive constituent (or cannabinoid) of cannabis. First isolated in 1964 by Israeli scientists Raphael Mechoulam and Yechiel Gaoni at the Weizmann Institute of Science it is a water-clear glassy solid when cold, which becomes viscous and sticky if warmed. A pharmaceutical formulation of (−)-trans-Δ9-tetrahydrocannabinol, known by its INN dronabinol, is available by prescription in the U.S. and Canada under the brand name Marinol. An aromatic terpenoid, THC has a very low solubility in water, but good solubility in most organic solvents, specifically lipids and alcohols. THC, CBD, CBN, CBC, CBG and about 80 other molecules make up the phytocannabinoid family. Like most pharmacologically-active secondary metabolites of plants, THC in Cannabis is assumed to be involved in self-defense, perhaps against herbivores. THC also possesses high UV-B (280–315 nm) absorption properties, which, it has been speculated, could protect the plant from harmful UV radiation exposure. Tetrahydrocannabinol, along with its double bond isomers and their stereoisomers, is one of only three cannabinoids scheduled by the Convention on Psychotropic Substances (the other two are dimethylheptylpyran and parahexyl). Cannabis as a plant is scheduled by the Single Convention on Narcotic Drugs (Schedule I and IV).