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: Fluoxetine , a selective serotonin reuptake inhibitor , is gaining increased acceptance in the treatment of adolescent depression .
### Output Text:
<span class="chemical">Fluoxetine</span> , a selective <span class="chemical">serotonin</span> reuptake inhibitor , is gaining increased acceptance in the treatment of adolescent depression . 