The server performs several BLAST searches(Altschul, et al., 1990) using each member of the first set as a query and the second set as the target database and vice versa.
This can take between 1 and 3 minutes depending on the size of the datasets and the server load.
Once the BLAST searches are completed, you can browse through the results using the "Prev" and "Next" buttons

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Selecting the box next to an entry will show the BLAST alignment

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You can filter the results according to various criteria, such as Similarity, E-value, coverage, etc ..