An amatoxin is any of a class of toxic peptides produced by several species of mushroom of the genus Amanita.
An amanitine is a bicyclic oligopeptide, found in Amanita phalloides, the death cap mushroom, which serves as a deadly amatoxin.
Amatoxins (found in many species of mushroom) inhibit RNA Polymerase - which is the enzyme responsible for transcribing (coping) DNA to mRNA. This means that protein synthesis cannot occur, causing cell death.