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: NS was induced by a single injection of puromycin amino - nucleoside ( PAN ) .
### Output Text:
NS was induced by a single injection of <span class="chemical">puromycin amino - nucleoside</span> ( <span class="chemical">PAN</span> ) . 