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: To study the interactions of these two genes on an organismal level , we bred mice heterozygous for null alleles of both atm and p53 to produce all genotypic combinations .
### Output Text:
To study the interactions of these two genes on an organismal level , we bred mice heterozygous for null alleles of both atm and p53 to produce all genotypic combinations . 