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 short variant of the polymorphism reduces the transcriptional efficiency of the 5 - HTT gene promoter , resulting in decreased 5 - HTT expression and 5 - HT uptake in lymphoblasts .
### Output Text:
The short variant of the polymorphism reduces the transcriptional efficiency of the 5 - HTT gene promoter , resulting in decreased 5 - HTT expression and 5 - HT uptake in lymphoblasts . 