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 instability of the CTG repeat appears to depend on its size as well as on the sex of the transmitting parent .
### Output Text:
The instability of the CTG repeat appears to depend on its size as well as on the sex of the transmitting parent . 