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: All but one of the 265 patients who were cured or improved at post - therapy returned for a long - term follow - up visit ; 243 ( 92 % ) remained well 4 to 6 weeks after therapy ; and 21 ( 8 % ) had a relapse of symptoms .
### Output Text:
All but one of the 265 patients who were cured or improved at post - therapy returned for a long - term follow - up visit ; 243 ( 92 % ) remained well 4 to 6 weeks after therapy ; and 21 ( 8 % ) had a relapse of symptoms . 