Metallothioneins are low molecular weight, cysteine-rich, heavy-metal binding proteins. They are present in some Prokaryota, fungi, plants and animals. Attempts to isolate native plant proteins failed but many metallothionein genes were identified in mono- and dicotyledons and also in some gymno- sperms and algae. They are organised in small gene families, like in animals genomes and their expression is upregulated by different endo- and exogenous factors, including metal ions.