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 mutation underlying Huntington disease ( HD ) is CAG expansion in the first exon of the HD gene .
### Output Text:
The mutation underlying <span class="disease">Huntington disease</span> ( <span class="disease">HD</span> ) is CAG expansion in the first exon of the <span class="disease">HD</span> gene . 