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: We therefore investigated the expression of VHL mRNA during human embryogenesis by in situ hybridization studies at 4 , 6 and 10 weeks post conception .
### Output Text:
We therefore investigated the expression of <span class="disease">VHL</span> mRNA during human embryogenesis by in situ hybridization studies at 4 , 6 and 10 weeks post conception . 