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: A subsequent search revealed a second gene mutation in HHUA cells , a missense mutation in the MSH6 gene .
### Output Text:
A subsequent search revealed a second gene mutation in HHUA cells , a missense mutation in the MSH6 gene . 