Protein tags are usually smallish peptides incorporated into a translated protein. As depicted in the accompanying cartoon, they have a multitude of uses including (but not limited to) purification, detection, solubilization, localization, or protease protection. Thus far Plasmids 101 has covered GFP and its related fluorescent proteins, which are sometimes used as tags for detection; however, those are just one (admittedly large) class of common fusion protein tags. Biochemists and molecular biologists who need to overexpress and purify proteins can face any number of technical challenges depending on their protein of interest. After several decades of trying to address these challenges, researchers have amassed a considerable molecular tool box of tags and fusion proteins to aid in the expression and purification of recombinant proteins.