24402
Moreover , the recent advent of knockout ( KO ) mice for tryptophan hydroxylase 2 ( Tph2 KO ) , which lacks the rate - limiting enzyme for 5 - HT synthesis in the brain , has further provided insight to the brain serotonergic system and its role in emotion dysregulation .
 Chemical: tryptophan; Chemical: 5 - HT
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: tryptophan; Chemical: 5 - HT