toxin
A chemical poison secreted by one organism for purposes of defense against a competing organism. Because toxins normally target highly specifi c cell/organ systems, many toxins have been used to gain insight into normal biochemical mechanisms; for example, tetrodotoxin, a toxin secreted by the puffer fi sh, spe- cifi cally paralyzes the sodium transport channel in nerves and has been used to study ion transport in the neural system.