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: Further studies are needed for a better clarification of Wilson ' s disease therapy , and in particular to differentiate specific therapies for different Wilson ' s disease phenotypes .
### Output Text:
Further studies are needed for a better clarification of Wilson ' s disease therapy , and in particular to differentiate specific therapies for different Wilson ' s disease phenotypes . 