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 human complement C9 gene : identification of two mutations causing deficiency and revision of the gene structure .
### Output Text:
The human complement C9 gene : identification of two mutations causing deficiency and revision of the gene structure . 