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 causes of heterogeneity are not known yet but are likely to include both different mutations at the SJS locus on chromosome 1 and the presence of a second SJS locus .
### Output Text:
The causes of heterogeneity are not known yet but are likely to include both different mutations at the <span class="disease">SJS</span> locus on chromosome 1 and the presence of a second <span class="disease">SJS</span> locus . 