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: We describe the largest group of AX - allergic patients who have tolerated PG reported so far .
### Output Text:
We describe the largest group of <span class="chemical">AX</span> - allergic patients who have tolerated <span class="chemical">PG</span> reported so far . 