6669
In humans , the disease phenotype is modified by a methionine / valine polymorphism at codon 129 of the prion protein gene .
 Chemical: methionine; Chemical: valine
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: methionine; Chemical: valine