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 report a G - - > A transition at nucleotide 431 of the proteolipid protein gene ( PLP ) results in a nonsense codon in a family with an unusual form of Pelizaeus - Merzbacher disease ( PMD ) .
### Output Text:
We report a G - - > A transition at nucleotide 431 of the proteolipid protein gene ( PLP ) results in a nonsense codon in a family with an unusual form of <span class="disease">Pelizaeus - Merzbacher disease</span> ( <span class="disease">PMD</span> ) . 