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: The present study investigates the effects of a diet free of precursors of NAD , the substrate on which PARP acts , in female NMRI mice with AAP hepatitis and evaluates the influence of simultaneous ethanol consumption in these animals .
### Output Text:
The present study investigates the effects of a diet free of precursors of <span class="chemical">NAD</span> , the substrate on which PARP acts , in female NMRI mice with <span class="chemical">AAP</span> hepatitis and evaluates the influence of simultaneous <span class="chemical">ethanol</span> consumption in these animals . 