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: Genetic linkage between SJS and chromosomal region 1q36 - 34 has been observed in several families , but the gene has not yet been identified .
### Output Text:
Genetic linkage between <span class="disease">SJS</span> and chromosomal region 1q36 - 34 has been observed in several families , but the gene has not yet been identified . 