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: Two were young patients whose chorea developed long after treatment had been started and disappeared soon after it had been discontinued .
### Output Text:
Two were young patients whose chorea developed long after treatment had been started and disappeared soon after it had been discontinued . 