TASK DEFINITION: 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?). 
PROBLEM: receptor usage , cellular source and biological activities.

SOLUTION: background

PROBLEM: and the expression of M fusion protein was induced by using isopropylthio-beta-D-galactoside ( IPTG ) as inducer.

SOLUTION: method

PROBLEM: We found that mice shed viruses distinct from input viruses because they formed extremely large plaques in cell culture.

SOLUTION:
finding