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: They are almost always benign , but can be mistaken for a serious adverse event by the patient .
### Output Text:
They are almost always benign , but can be mistaken for a serious adverse event by the patient . 