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: Regulation of transcript elongation is an important control mechanism for gene expression and the VHL gene might modify the expression of proto - oncogenes and growth suppressor genes during embryogenesis .
### Output Text:
Regulation of transcript elongation is an important control mechanism for gene expression and the <span class="disease">VHL</span> gene might modify the expression of proto - oncogenes and growth suppressor genes during embryogenesis . 