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: Because 5 - HT transporters play a key element in the regulation of synaptic 5 - HT transmission it may be important to control for the potential covariance effect of a polymorphism in the 5 - HT transporter promoter gene region ( 5 - HTTLPR ) when studying the effects of MDMA as well as cognitive functioning .
### Output Text:
Because <span class="chemical">5 - HT</span> transporters play a key element in the regulation of synaptic <span class="chemical">5 - HT</span> transmission it may be important to control for the potential covariance effect of a polymorphism in the <span class="chemical">5 - HT</span> transporter promoter gene region ( 5 - HTTLPR ) when studying the effects of <span class="chemical">MDMA</span> as well as cognitive functioning . 