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: However , physicians should be aware of the possibility that mirtazapine can be associated with RLS in some individuals , especially those receiving concomitant dopamine D2 receptor antagonists .
### Output Text:
However , physicians should be aware of the possibility that <span class="chemical">mirtazapine</span> can be associated with RLS in some individuals , especially those receiving concomitant <span class="chemical">dopamine</span> D2 receptor antagonists . 