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: Furthermore , it appears that striatal - resident microglia respond to METH with an activation cascade and then return to a surveying state without undergoing apoptosis or migration .
### Output Text:
Furthermore , it appears that striatal - resident microglia respond to <span class="chemical">METH</span> with an activation cascade and then return to a surveying state without undergoing apoptosis or migration . 