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: PATIENTS AND METHODS : Fifty - one assessable patients , 2 to 12 years of age , were treated with oral cis - RA administered in two equally divided doses daily for 2 weeks , followed by a 2 - week rest period , for up to 12 courses .
### Output Text:
PATIENTS AND METHODS : Fifty - one assessable patients , 2 to 12 years of age , were treated with oral <span class="chemical">cis - RA</span> administered in two equally divided doses daily for 2 weeks , followed by a 2 - week rest period , for up to 12 courses . 