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 nonsense mutation , which is in exon 3B , should block the synthesis of normal PLP but spare DM20 , the isoform whose persistence has been associated with mild forms of PLP - associated disease in both humans and mice . .
### Output Text:
The nonsense mutation , which is in exon 3B , should block the synthesis of normal PLP but spare DM20 , the isoform whose persistence has been associated with mild forms of <span class="disease">PLP - associated disease</span> in both humans and mice . . 