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: Receptor binding studies confirmed that UMB24 and ( + / - ) - SM 21 display preferential affinity for sigma2 over sigma1 receptors .
### Output Text:
Receptor binding studies confirmed that <span class="chemical">UMB24</span> and ( + / - ) - <span class="chemical">SM 21</span> display preferential affinity for sigma2 over sigma1 receptors . 