Metallothioneins (MTs) are a family of low molecular weight, heavy metal-binding proteins characterized by a high cysteine content and lack of aromatic amino acids. MTs bind 7 to 12 heavy metal atoms per molecule of protein. They are ubiquitous in the animal and plant kingdoms and are also found in prokaryotes. In mammals, the cysteine residues are absolutely conserved and serve to coordinate heavy metal atoms such as zinc, cadmium, and copper via mercaptide linkages.