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 low incidence of somatic mutations suggests that BRCA1 inactivation in sporadic tumors occurs by alternative mechanisms , such as interstitial chromosomal deletion or reduced transcription .
### Output Text:
The low incidence of somatic mutations suggests that BRCA1 inactivation in sporadic <span class="disease">tumors</span> occurs by alternative mechanisms , such as interstitial chromosomal deletion or reduced transcription . 