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: X - chromosome inactivation in mammals is regarded as an essentially random process , but the resulting somatic - cell mosaicism creates the opportunity for cell selection .
### Output Text:
X - chromosome inactivation in mammals is regarded as an essentially random process , but the resulting somatic - cell mosaicism creates the opportunity for cell selection . 