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: Influence of diet free of NAD - precursors on acetaminophen hepatotoxicity in mice .
### Output Text:
Influence of diet free of <span class="chemical">NAD</span> - precursors on <span class="chemical">acetaminophen</span> hepatotoxicity in mice . 