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: Limitations of the current dietary treatment for PKU have led to the development of potential treatments based on somatic gene transfer .
### Output Text:
Limitations of the current dietary treatment for <span class="disease">PKU</span> have led to the development of potential treatments based on somatic gene transfer . 