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: BACKGROUND & AIMS The chromosome region 18q21 has been shown to be frequently deleted in colorectal cancers , and such frequent allelic loss is a hallmark of the presence of a tumor - suppressor gene .
### Output Text:
BACKGROUND & AIMS The chromosome region 18q21 has been shown to be frequently deleted in <span class="disease">colorectal cancers</span> , and such frequent allelic loss is a hallmark of the presence of a tumor - suppressor gene . 