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: Of the patients developing one or more recurrences during the first year , only 50 % presented with further recurrence once the instillations were stopped .
### Output Text:
Of the patients developing one or more recurrences during the first year , only 50 % presented with further recurrence once the instillations were stopped . 