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: BACKGROUND : In recent studies increased amounts of nitric oxide ( NO ) and apoptosis have been implicated in various pathological conditions in the kidney .
### Output Text:
BACKGROUND : In recent studies increased amounts of <span class="chemical">nitric oxide</span> ( <span class="chemical">NO</span> ) and apoptosis have been implicated in various pathological conditions in the kidney . 