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: NRA0160 is over 20 , 000fold more potent at the dopamine D4 . 2 receptor compared with the human cloned dopamine D2L receptor .
### Output Text:
<span class="chemical">NRA0160</span> is over 20 , 000fold more potent at the <span class="chemical">dopamine</span> D4 . 2 receptor compared with the human cloned <span class="chemical">dopamine</span> D2L receptor . 