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?). 

SpyTag ( A ) , and SpyCatcher ( B ).
background
The sentence introduces two proteins that were previously shown to be involved in the formation of amyloid fibers. The purpose of this study is to investigate their function as a protein tag and an anti-tag, respectively, in different cellular settings. Therefore, the sentence describes relevant previous work that is related to the research purpose and provides background information about the topic. So it belongs to the "Background" category..