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: According to annexin V binding , erythrocytes from patients indeed showed a significant increase of PS exposure within 1 week of treatment with azathioprine .
### Output Text:
According to annexin V binding , erythrocytes from patients indeed showed a significant increase of <span class="chemical">PS</span> exposure within 1 week of treatment with <span class="chemical">azathioprine</span> . 