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: Combined subtotal deficiency of C6 and C7 , in which both proteins are expressed at very low levels , has been observed in homozygous form in two families .
### Output Text:
<span class="disease">Combined subtotal deficiency of C6 and C7</span> , in which both proteins are expressed at very low levels , has been observed in homozygous form in two families . 