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: In the present study , we have observed mutations involving codon 50 in 11 of 37 pancreatic tumors .
### Output Text:
In the present study , we have observed mutations involving codon 50 in 11 of 37 <span class="disease">pancreatic tumors</span> . 