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: Since most of Wilson ' s disease penicillamine - treated patients do not seem to develop this skin lesion , it could be conceivable that a specific genetic factor is involved in drug response .
### Output Text:
Since most of Wilson ' s disease <span class="chemical">penicillamine</span> - treated patients do not seem to develop this skin lesion , it could be conceivable that a specific genetic factor is involved in drug response . 