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: Using Northern and Western blot analysis to investigate the tissue specific distribution of VLCAD mRNA and protein in several human tissues we showed that VLCAD is most abundant in heart and skeletal muscle .
### Output Text:
Using Northern and Western blot analysis to investigate the tissue specific distribution of VLCAD mRNA and protein in several human tissues we showed that VLCAD is most abundant in heart and skeletal muscle . 