In the sentence extracted from biomedical literature, identify all the disease entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="disease"> to denote a disease.

### Input Text: The rate of cleavage increases with the length of the huntingtin polyglutamine tract , providing an explanation for the gain - of - function associated with CAG expansion .
### Output Text:
The rate of cleavage increases with the length of the huntingtin polyglutamine tract , providing an explanation for the gain - of - function associated with CAG expansion . 