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: Taken together , these findings demonstrate that DA is not essential for the development of METH - induced dopaminergic neurotoxicity and suggest that mechanisms independent of DA warrant more intense investigation .
### Output Text:
Taken together , these findings demonstrate that <span class="chemical">DA</span> is not essential for the development of <span class="chemical">METH</span> - induced dopaminergic neurotoxicity and suggest that mechanisms independent of <span class="chemical">DA</span> warrant more intense investigation . 