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: RESULTS : A 2 . 0 - mg oral dose of trihexyphenidyl resulted in increased subjective ratings of mental slowness in carriers of the APOE epsilon4 allele only .
### Output Text:
RESULTS : A 2 . 0 - mg oral dose of <span class="chemical">trihexyphenidyl</span> resulted in increased subjective ratings of mental slowness in carriers of the APOE epsilon4 allele only . 