12943
Here we report that Rif1 ( - / - ) mice are severely compromised for 53BP1 - dependent class switch recombination ( CSR ) and fusion of dysfunctional telomeres .
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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:
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