22643
Research in this field has been substantially promoted by the evidence that the reuptake protein ( SERT ) , present in presynaptic neurons , is a key element in terminating the activity of the neurotransmitter in the synaptic cleft .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
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