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 , the behavioral effects of centrally administered U - II have received little attention .
### Output Text:
However , the behavioral effects of centrally administered <span class="chemical">U - II</span> have received little attention . 