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 acetylcholine receptor antibody titer and response to acetylcholinesterase inhibitors were negative , and the results of thyroid function tests were normal .
### Output Text:
The <span class="chemical">acetylcholine</span> receptor antibody titer and response to acetylcholinesterase inhibitors were negative , and the results of thyroid function tests were normal . 