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The nociceptin / orphanin FQ ( N / OFQ ) peptide ( NOP ) receptor is the fourth and most recently discovered member of the opioid receptor superfamily that also includes mu , delta , and kappa opioid receptor subtypes ( MOR , DOR , and KOR , respectively ) .
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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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