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: Molecular analysis of the patients DNA showed a single base ( G ) deletion involving the GT dinucleotide at the start of intron 18 destroying a splice site of the COL2A1 gene .
### Output Text:
Molecular analysis of the patients DNA showed a single base ( G ) deletion involving the GT dinucleotide at the start of intron 18 destroying a splice site of the COL2A1 gene . 