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: Only a few reports exist , however , on the relationship between the serum concentrations of selective serotonin reuptake inhibitors ( SSRIs ) and their toxic effects .
### Output Text:
Only a few reports exist , however , on the relationship between the serum concentrations of selective <span class="chemical">serotonin</span> reuptake inhibitors ( SSRIs ) and their toxic effects . 