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: Memory function and serotonin transporter promoter gene polymorphism in ecstasy ( MDMA ) users .
### Output Text:
Memory function and <span class="chemical">serotonin</span> transporter promoter gene polymorphism in <span class="chemical">ecstasy</span> ( <span class="chemical">MDMA</span> ) users . 