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: MEK1 was significantly decreased also after a single injection of METH , but to a much lesser degree than after multiple injections of METH .
### Output Text:
MEK1 was significantly decreased also after a single injection of <span class="chemical">METH</span> , but to a much lesser degree than after multiple injections of <span class="chemical">METH</span> . 