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: Twenty - nine of them were later excluded because of adverse effects of PTU or because the observation period was less than 3 months .
### Output Text:
Twenty - nine of them were later excluded because of adverse effects of <span class="chemical">PTU</span> or because the observation period was less than 3 months . 