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: Improvement of humoral immunity occurred in 60 % of responders ( p < 0 . 001 ) and in the majority of patients who achieved stable disease .
### Output Text:
Improvement of humoral immunity occurred in 60 % of responders ( p < 0 . 001 ) and in the majority of patients who achieved stable disease . 