In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: We did not find abnormalities in the expression and distribution of an mtDNA - encoded subunit of CCO ( CCO - I ) or a relative decrease in CCO - I when compared with nuclear - encoded subunits ( CCO - IV and SDH - fp ) .
### Output Text:
We did not find abnormalities in the expression and distribution of an mtDNA - encoded subunit of CCO ( CCO - I ) or a relative decrease in CCO - I when compared with nuclear - encoded subunits ( CCO - IV and SDH - fp ) . 