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 quantitative distribution of VHL mRNA within fetal tissues reflected that seen by in situ hybridization and the ratio of the two VHL isoforms was similar between tissues .
### Output Text:
The quantitative distribution of <span class="disease">VHL</span> mRNA within fetal tissues reflected that seen by in situ hybridization and the ratio of the two <span class="disease">VHL</span> isoforms was similar between tissues . 