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 deficiency is characterized by prolonged apnea after the use of muscle relaxants ( suxamethonium or mivacurium ) in patients who have mutations in the BCHE gene .
### Output Text:
Butyrylcholinesterase deficiency is characterized by prolonged apnea after the use of muscle relaxants ( <span class="chemical">suxamethonium</span> or <span class="chemical">mivacurium</span> ) in patients who have mutations in the BCHE gene . 