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: Because each patient was tumor - free or had only a small tumor at the onset of this syndrome , the thrombotic microangiopathy may have been induced by chemotherapy .
### Output Text:
Because each patient was tumor - free or had only a small tumor at the onset of this syndrome , the thrombotic microangiopathy may have been induced by chemotherapy . 