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: PURPOSE : Treatment of neuroblastoma cell lines with 13 - cis - retinoic acid ( cis - RA ) can cause sustained inhibition of proliferation .
### Output Text:
PURPOSE : Treatment of neuroblastoma cell lines with <span class="chemical">13 - cis - retinoic acid</span> ( <span class="chemical">cis - RA</span> ) can cause sustained inhibition of proliferation . 