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: However , it is not yet clear what proportion of hereditary breast cancer is explained by BRCA1 and BRCA2 or by some other unidentified susceptibility gene ( s ) .
### Output Text:
However , it is not yet clear what proportion of <span class="disease">hereditary breast cancer</span> is explained by BRCA1 and BRCA2 or by some other unidentified susceptibility gene ( s ) . 