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: The duration of action may be prolonged in patients with genetic variants of the butyrylcholinesterase enzyme ( BChE ) , the most common being the K - and the A - variants .
### Output Text:
The duration of action may be prolonged in patients with genetic variants of the butyrylcholinesterase enzyme ( BChE ) , the most common being the K - and the A - variants . 