Amanitin is the most powerful poison found in Amanita mushrooms. Its lethal dose is only 0.1-0.3 mg/kg, meaning that only 10 mg will kill a 100 kg man. Amanitin(alpha-amanitin, to be exact) has an unusually strong and specific attraction to the enzyme RNA polymerase II, so upon ingestion, the poison binds to this enzyme, killing liver cells. See http://en.wikipedia.org/wiki/Alpha-amanitin Amanita phalloides(Death cup)

Other reference: Berkow, Robert(editor)The Merck Manual 16th edition. 1992 Amanita muscaria