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 apparently healthy survival into old age of some individuals with 36 - 39 repeats suggests that the HD mutation may not always be fully penetrant . .
### Output Text:
The apparently healthy survival into old age of some individuals with 36 - 39 repeats suggests that the <span class="disease">HD</span> mutation may not always be fully penetrant . . 