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: After 30 days of CY injection ulcerous and non - ulcerous forms of chronic cystitis appeared in 86 % of DBA / 2 mice but only in 4 % of C57BL / 6 mice .
### Output Text:
After 30 days of <span class="chemical">CY</span> injection ulcerous and non - ulcerous forms of chronic cystitis appeared in 86 % of DBA / 2 mice but only in 4 % of C57BL / 6 mice . 