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: Toxicities were compared with a cohort of patients in a phase I trial of paclitaxel alone at identical dose levels .
### Output Text:
Toxicities were compared with a cohort of patients in a phase I trial of <span class="chemical">paclitaxel</span> alone at identical dose levels . 