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: RAST was positive for AX in 22 patients ( 41 % ) and to BPO in just 5 ( 9 % ) .
### Output Text:
RAST was positive for <span class="chemical">AX</span> in 22 patients ( 41 % ) and to <span class="chemical">BPO</span> in just 5 ( 9 % ) . 