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: Somatic - cell selection is a major determinant of the blood - cell phenotype in heterozygotes for glucose - 6 - phosphate dehydrogenase mutations causing severe enzyme deficiency .
### Output Text:
Somatic - cell selection is a major determinant of the blood - cell phenotype in heterozygotes for glucose - 6 - phosphate dehydrogenase mutations causing severe <span class="disease">enzyme deficiency</span> . 