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: It is important to recognize the rare and potentially reversible toxicity of AmB .
### Output Text:
It is important to recognize the rare and potentially reversible toxicity of <span class="chemical">AmB</span> . 