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: CONCLUSION : eleven of 13 patients with a prolonged duration of action of succinylcholine had mutations in BCHE , indicating that this is the possible reason for a prolonged period of apnea .
### Output Text:
CONCLUSION : eleven of 13 patients with a prolonged duration of action of <span class="chemical">succinylcholine</span> had mutations in BCHE , indicating that this is the possible reason for a prolonged period of apnea . 