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: In contrast , cells expressing the mutant fragment secreted a monomeric molecule of apparent molecular mass of 80 kDa , which remained unchanged after reduction .
### Output Text:
In contrast , cells expressing the mutant fragment secreted a monomeric molecule of apparent molecular mass of 80 kDa , which remained unchanged after reduction . 