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: When administered alone , ( + / - ) - SM 21 produced no significant effects compared to control injections of saline , but UMB24 had locomotor depressant actions .
### Output Text:
When administered alone , ( + / - ) - <span class="chemical">SM 21</span> produced no significant effects compared to control injections of saline , but <span class="chemical">UMB24</span> had locomotor depressant actions . 