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: ATM + / - fibroblasts and thymocytes showed intermediately defective responses to irradiation but no growth defect , suggesting that the increased cancer risk of AT heterozygotes could be attributable to poor checkpoint function . .
### Output Text:
ATM + / - fibroblasts and thymocytes showed intermediately defective responses to irradiation but no growth defect , suggesting that the increased <span class="disease">cancer</span> risk of <span class="disease">AT</span> heterozygotes could be attributable to poor checkpoint function . . 