8649
Among opioid receptor genes , one polymorphism is much more frequent in human populations than the many others that have been found , but the functional significance of this single nucleotide polymorphism ( SNP ) has been unclear .
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Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
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