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 effectiveness of PTRA on renal function seems to be barely influenced by CM toxicity .
### Output Text:
The effectiveness of PTRA on renal function seems to be barely influenced by <span class="chemical">CM</span> toxicity . 