4846
In situ hybridization experiments showed that TCDD exposure also prevented the expression of the PE marker tcf21 at the site where the PE normally forms .
 Chemical: TCDD
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
 Entities: TCDD 
Answer:
 Chemical: TCDD