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: Effects of UMB24 and ( + / - ) - SM 21 , putative sigma2 - preferring antagonists , on behavioral toxic and stimulant effects of cocaine in mice .
### Output Text:
Effects of <span class="chemical">UMB24</span> and ( + / - ) - <span class="chemical">SM 21</span> , putative sigma2 - preferring antagonists , on behavioral toxic and stimulant effects of <span class="chemical">cocaine</span> in mice . 