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: CIN was defined as an increase in serum creatinine ( Cr ) of 0 . 5 mg / dl or more , or elevation of Cr to 25 % over baseline .
### Output Text:
CIN was defined as an increase in serum <span class="chemical">creatinine</span> ( <span class="chemical">Cr</span> ) of 0 . 5 mg / dl or more , or elevation of <span class="chemical">Cr</span> to 25 % over baseline . 