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: A 67 - year - old patient , with primary polymyositis and without previous evidence of liver disease , developed clinical and biochemical features of severe cholestasis 3 months after initiation of azathioprine therapy .
### Output Text:
A 67 - year - old patient , with primary polymyositis and without previous evidence of liver disease , developed clinical and biochemical features of severe cholestasis 3 months after initiation of <span class="chemical">azathioprine</span> therapy . 