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 50 patients evaluable for response , 12 ( 24 % ) had partial remission , 26 ( 52 % ) had no change , and 12 had disease progression ( 24 % ) .
### Output Text:
Of 50 patients evaluable for response , 12 ( 24 % ) had partial remission , 26 ( 52 % ) had no change , and 12 had disease progression ( 24 % ) . 