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Excessive level of estrogen is considered as a main cause of breast cancer , therefore , many studies have focused on estrogen receptor ( ER ) - positive breast cancer , even though ER - negative cancer has a poor prognosis than ER - positive breast cancer .
 Chemical: estrogen; Chemical: estrogen
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: estrogen; Chemical: estrogen