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 wide spectrum of mutations , ranging from point mutations to large deletions , have been described in the retinoblastoma gene ( RB1 ) .
### Output Text:
A wide spectrum of mutations , ranging from point mutations to large deletions , have been described in the <span class="disease">retinoblastoma</span> gene ( RB1 ) . 