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: Where it is combined with a C6 - deficient gene , the serum C7 levels can be surprisingly high , possibly because there is no C6 generating C56 to consume the C7 .
### Output Text:
Where it is combined with a <span class="disease">C6 - deficient</span> gene , the serum C7 levels can be surprisingly high , possibly because there is no C6 generating C56 to consume the C7 . 