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: The 1721C - - > T transition is a neutral polymorphism for leucine at amino acid 218 ( L218L ) .
### Output Text:
The 1721C - - > T transition is a neutral polymorphism for leucine at amino acid 218 ( L218L ) . 