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 much larger fraction of breast cancer might , in principle , be due to common variants which confer more modest individual risks .
### Output Text:
A much larger fraction of <span class="disease">breast cancer</span> might , in principle , be due to common variants which confer more modest individual risks . 