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: Eight novel and three previously characterized mutations accounted for 80 % ( 40 / 50 ) of mutant chromosomes .
### Output Text:
Eight novel and three previously characterized mutations accounted for 80 % ( 40 / 50 ) of mutant chromosomes . 