Metallothioneins (MTs) are ubiquitous low molecular weight proteins characterized by their abundant content of cysteines. Two MT isoforms, MT-I and MT-II, are expressed coordinately in all mammalian tissues. In the CNS, MT-I and MT-II are conspicuously absent from neuronal populations, yet abundant in fibrous and protoplasmic astrocytes. A newly identified brain-specific MT gene, MT-III, is predominantly expressed in zinc-containing neurons of the hippocampus and absent from glial elements. MTs have been implicated as regulator molecules in gene expression, homeostatic control of cellular metabolism of metals, and cellular adaptation to stress. MTs store and release essential metals, such as zinc and copper, maintaining the low intracellular concentration of free essential metals. Thus, MTs fulfill a regulatory capacity and influence transcription, replication, protein synthesis, metabolism, as well as other zinc-dependent biological processes. Because MT-III is particularly abundant in zinc-containing ne...