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 skin tests revealed positive delayed reactions of 24 hours and 48 hours by IDR and patch tests to only some PRC with common chains in their structures .
### Output Text:
The skin tests revealed positive delayed reactions of 24 hours and 48 hours by IDR and patch tests to only some <span class="chemical">PRC</span> with common chains in their structures . 