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 present study was done to investigate the protective effect of TCR on experimentally induced myocardial infarction in rats .
### Output Text:
The present study was done to investigate the protective effect of <span class="chemical">TCR</span> on experimentally induced myocardial infarction in rats . 