Metallothioneins have diverse metal-binding preferences, which have been associated with functional specificity. As an example, the mammalian "Mus musculus" MT1 preferentially binds divalent metal ions (Zn(II), Cd(II)...), while yeast CUP1 is selective for monovalent metal ions (Cu(I), Ag(I)...). Strictly metal-selective MTs with metal-specific physiological functions were discovered by Dallinger et al. (1997) in pulmonate snails (Gastropoda, Mollusca). The Roman snail ("Helix pomatia"), for example, possesses a Cd-selective (CdMT) and a Cu-selective isoform (CuMT) involved in Cd detoxification and Cu regulation, respectively. While both isoforms contain unvaried numbers and positions of Cys residues responsible for metal ligation, metal selectivity is apparently achieved by sequence modulation of amino acid residues not directly involved in metal binding (Palacios et al. 2011).