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: In the diagnostic setting it is necessary to define the limits of the CAG size ranges on normal and HD - associated chromosomes .
### Output Text:
In the diagnostic setting it is necessary to define the limits of the CAG size ranges on normal and <span class="disease">HD</span> - associated chromosomes . 