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: Many of these genes are regulated by the peroxisome proliferator - activated receptor - alpha ( PPARalpha ) , a ligand - activated nuclear hormone receptor transcription factor .
### Output Text:
Many of these genes are regulated by the peroxisome proliferator - activated receptor - alpha ( PPARalpha ) , a ligand - activated nuclear hormone receptor transcription factor . 