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 Prof. Raphael Mechoulam and Dr. 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.