In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: We therefore hypothesized that overexpression of Cu / Zn - superoxide dismutase ( h - SOD1 ) should protect transgenic mice from ototoxicity .
### Output Text:
We therefore hypothesized that overexpression of <span class="chemical">Cu</span> / <span class="chemical">Zn</span> - <span class="chemical">superoxide</span> dismutase ( h - SOD1 ) should protect transgenic mice from ototoxicity . 