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: Although 3 , 4 - methylenedioxymethamphetamine ( MDMA or ecstasy ) has been shown to damage brain serotonin ( 5 - HT ) neurons in animals and possibly humans , little is known about the long - term consequences of MDMA - induced 5 - HT neurotoxic lesions on functions in which 5 - HT is involved , such as cognitive function .
### Output Text:
Although <span class="chemical">3 , 4 - methylenedioxymethamphetamine</span> ( <span class="chemical">MDMA</span> or <span class="chemical">ecstasy</span> ) has been shown to damage brain <span class="chemical">serotonin</span> ( <span class="chemical">5 - HT</span> ) neurons in animals and possibly humans , little is known about the long - term consequences of <span class="chemical">MDMA</span> - induced <span class="chemical">5 - HT</span> neurotoxic lesions on functions in which <span class="chemical">5 - HT</span> is involved , such as cognitive function . 