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: To investigate if the two isoforms are differentially expressed during embryogenesis , VHL mRNA was reverse transcribed from 13 fetal tissues ( 8 - 10 weeks gestation ) .
### Output Text:
To investigate if the two isoforms are differentially expressed during embryogenesis , <span class="disease">VHL</span> mRNA was reverse transcribed from 13 fetal tissues ( 8 - 10 weeks gestation ) . 