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 24 exons of BRCA1 span an 81 - kb region that has an unusually high density of Alu repetitive DNA ( 41 . 5 % ) , but relatively low density ( 4 . 8 % ) of other repetitive sequences .
### Output Text:
The 24 exons of BRCA1 span an 81 - kb region that has an unusually high density of Alu repetitive DNA ( 41 . 5 % ) , but relatively low density ( 4 . 8 % ) of other repetitive sequences . 