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: Although a great urinary excretion and half - normal plasma levels of Ao were observed on day 6 after PAN injection , when NS was clearly established , hepatic Ao mRNA levels did not change .
### Output Text:
Although a great urinary excretion and half - normal plasma levels of Ao were observed on day 6 after <span class="chemical">PAN</span> injection , when NS was clearly established , hepatic Ao mRNA levels did not change . 