Metallothioneins constitute a family of cysteine-rich, low molecular weight metal-binding proteins. They bind physiological metals, such as zinc and copper, as well as nonessential, toxic heavy metals, such as cadmium, mercury, and silver. They play an essential role in heavy metal detoxification and in maintaining the homeostasis of essential trace metals. They are also involved in the protection against free radicals and oxidative stress. (Adapted from FBrf0215591).