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: Phase I trial of 13 - cis - retinoic acid in children with neuroblastoma following bone marrow transplantation .
### Output Text:
Phase I trial of <span class="chemical">13 - cis - retinoic acid</span> in children with neuroblastoma following bone marrow transplantation . 