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Given the high heritability of MD ( estimated between 31 % and 42 % for unipolar depression ) , genes coding for key regulators of the serotonergic neurotransmission have been considered as optimal candidates .
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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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