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 order to test this hypothesis , we have studied samples of single sperm from three individuals heterozygous at the DM locus , each with one allele larger and one allele smaller than 19 CTG repeats .
### Output Text:
In order to test this hypothesis , we have studied samples of single sperm from three individuals heterozygous at the <span class="disease">DM</span> locus , each with one allele larger and one allele smaller than 19 CTG repeats . 