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 have designed high - density arrays consisting of over 96 , 600 oligonucleotides 20 - nucleotides ( nt ) in length to screen for a wide range of heterozygous mutations in the 3 .
### Output Text:
We have designed high - density arrays consisting of over 96 , 600 oligonucleotides 20 - nucleotides ( nt ) in length to screen for a wide range of heterozygous mutations in the 3 . 