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 Registry for Liver Tumors Associated with Oral Contraceptives at the University of California , Irvine , has clearly identified 27 cases .
### Output Text:
The Registry for Liver Tumors Associated with <span class="chemical">Oral Contraceptives</span> at the University of California , Irvine , has clearly identified 27 cases . 