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 level of the 137 - bp PCR product containing the insertion was lowest in two patients who showed a later onset of cerebellar ataxia .
### Output Text:
The level of the 137 - bp PCR product containing the insertion was lowest in two patients who showed a later onset of <span class="disease">cerebellar ataxia</span> . 