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: Human 5 - HT transporter ( 5 - HTT ) gene transcription is modulated by a common polymorphism in its upstream regulatory region .
### Output Text:
Human 5 - HT transporter ( 5 - HTT ) gene transcription is modulated by a common polymorphism in its upstream regulatory region . 