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: RNA in situ hybridization revealed expression in retina , brain and the olfactory bulb and epithelium of 2 week old mice .
### Output Text:
RNA in situ hybridization revealed expression in retina , brain and the olfactory bulb and epithelium of 2 week old mice . 