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: RESULTS : Exposure to azathioprine ( > or = 2 microg / mL ) for 48 hours increased cytosolic Ca2 + activity and annexin V binding and decreased forward scatter .
### Output Text:
RESULTS : Exposure to <span class="chemical">azathioprine</span> ( > or = 2 microg / mL ) for 48 hours increased cytosolic <span class="chemical">Ca2</span> + activity and annexin V binding and decreased forward scatter . 