Metallothioneins (MT) are the widespread proteins in animal world. These proteins are characterized by the great invariability of their structure. While isolated from the different organs of different animals they only slightly differ in the aminoacid composition from one another. Studies on the structure and biological functions of MT have identified four major isoforms (MT-I, -II, -III and ?IV) in mammals. Both MT-I and MT-II genes are expressed in various organs and they are inducible by a variety of metals, hormones, cytokines and xenobiotics. MT-III is expressed only in brain and no specific environmental regulatores have been identified. MT-IV is only expressed in differentiating cells of stratified squamous epithelia.