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 guard against the possible problem of differential PCR amplification rates based on the lengths of the alleles , the sperm were also typed at another closely linked marker whose allele size was unrelated to the allele size at the DM locus .
### Output Text:
To guard against the possible problem of differential PCR amplification rates based on the lengths of the alleles , the sperm were also typed at another closely linked marker whose allele size was unrelated to the allele size at the <span class="disease">DM</span> locus . 