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: None of the sera with negative RAST for AX were positive to BPO .
### Output Text:
None of the sera with negative RAST for <span class="chemical">AX</span> were positive to <span class="chemical">BPO</span> . 