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: Low activity of patient plasma butyrylcholinesterase with butyrylthiocholine ( BTC ) and benzoylcholine , and values of dibucaine and fluoride numbers fit with heterozygous atypical silent genotype .
### Output Text:
Low activity of patient plasma butyrylcholinesterase with <span class="chemical">butyrylthiocholine</span> ( <span class="chemical">BTC</span> ) and <span class="chemical">benzoylcholine</span> , and values of <span class="chemical">dibucaine</span> and <span class="chemical">fluoride</span> numbers fit with heterozygous atypical silent genotype . 