Protects the cell against copper toxicity by tightly chelating copper ions. May also act as a depository for copper designated for the effective transfer into the apo forms of copper proteins. Contains 1 metal-binding domain: 6 to 8 copper ions are chelated within a single copper-thiolate cluster and are coordinated via cysteinyl thiolate bridges to 10 cysteine ligands. 6 copper ions are trigonally coordinated, whereas the other 2 are only digonally coordinated. There are 2 copies for copper thionein in yeast. The 2 identical copies CUP1-1 and CUP1-2 are arranged in tandem. Belongs to the metallothionein superfamily. Type 12 family.