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However , we also found that not all nitroreductases express stably in human ( HCT - 116 colon carcinoma ) cells , and that activity at a purified protein level did not necessarily predict activity in stably transfected HCT - 116 .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 None