3969
They vary highly in activities : along the length of the intestine and along the villi , gradients of expression levels of the enzymes and proteins exist , which necessitates an in vitro model that can reflect the different regions of the intestine .
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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:
 None