In this task, you are given a sentence from the research paper and your task is to classify the given sentence into the following categories: Background (Why is this problem important? What relevant works have been created before? What is still missing in the previous works? What are the high-level research questions? How might this help other research or researchers?), Purpose (What specific things do the researchers want to do? What specific knowledge do the researchers want to gain? What specific hypothesis do the researchers want to test?), Method (How did the researchers do the work or find what they sought? What are the procedures and steps of the research?), or Finding (What did the researchers find out? Did the proposed methods work? Did the thing behave as the researchers expected?). 

Q: Male PTX3 knockout ( KO ) mice and their wild type ( WT ) littermates on 129/SvEv/C57BL6 / J background were used as recipients.

A: method
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Q: a cytotoxin that causes leukocyte destruction and tissue necrosis ( 1 ).

A: background
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Q: discovery and structureeactivity relationship studies of the coumarin analogues as antiviral agents focusing mainly on lead optimization and its development into clinical candidates.

A:
purpose
****