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: The expression analysis of apoptosis - related proteins revealed that pro - apoptotic protein expression was upregulated , and anti - apoptotic protein BCL - 2 expression was downregulated .
### Output Text:
The expression analysis of apoptosis - related proteins revealed that pro - apoptotic protein expression was upregulated , and anti - apoptotic protein BCL - 2 expression was downregulated . 