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: METHODS Mutation analyses of the DPC4 gene were performed on complementary DNA samples from 31 primary colorectal cancer specimens using a combination of polymerase chain reaction , single - strand conformation polymorphism , and DNA sequencing .
### Output Text:
METHODS Mutation analyses of the DPC4 gene were performed on complementary DNA samples from 31 primary <span class="disease">colorectal cancer</span> specimens using a combination of polymerase chain reaction , single - strand conformation polymorphism , and DNA sequencing . 