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: Since cis - RA has demonstrated clinical responses in neuroblastoma patients , it may be effective in preventing relapse after cytotoxic therapy .
### Output Text:
Since <span class="chemical">cis - RA</span> has demonstrated clinical responses in neuroblastoma patients , it may be effective in preventing relapse after cytotoxic therapy . 