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: We therefore conclude that the HGO cDNA encodes the gene responsible for alkaptonuria .
### Output Text:
We therefore conclude that the HGO cDNA encodes the gene responsible for <span class="disease">alkaptonuria</span> . 