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 conclude from these studies that CX3CR1 signaling does not modulate METH neurotoxicity or microglial activation .
### Output Text:
We conclude from these studies that CX3CR1 signaling does not modulate <span class="chemical">METH</span> neurotoxicity or microglial activation . 