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: Liver biopsies should be undertaken at regular intervals if azathioprine therapy is continued so that structural liver damage may be detected at an early and reversible stage .
### Output Text:
Liver biopsies should be undertaken at regular intervals if <span class="chemical">azathioprine</span> therapy is continued so that structural liver damage may be detected at an early and reversible stage . 