One of the most common bioinformatics analyses is to identify proteins that are homologous to a sequence of interest (soi). Sometimes the soi is a novel protein from a new genome, and we want to identify homologs in order to predict its function. In other cases, we may want to know how widespread proteins with a specific function are. In this chapter, we search for homologs of proteins that cause resistance to colistin, which was until recently one of the few antibiotics without widespread resistance (no longer true, unfortunately). Student should have chosen an aoi in Chaps. 2 and 3 , and use the same approach to identify homologs of your aoi. We will use basic local alignment search tool (BLAST) to identify homologs of your aoi.