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: Butyrylcholinesterase gene mutations in patients with prolonged apnea after succinylcholine for electroconvulsive therapy .
### Output Text:
Butyrylcholinesterase gene mutations in patients with prolonged apnea after <span class="chemical">succinylcholine</span> for electroconvulsive therapy . 