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: In Northern blot analysis , an mRNA of over 10 kb was detected by these ectopic sequences in a variety of human tissues but not in brain and peripheral blood leukocytes .
### Output Text:
In Northern blot analysis , an mRNA of over 10 kb was detected by these ectopic sequences in a variety of human tissues but not in brain and peripheral blood leukocytes . 