Fusion tags (orchid) are short peptides, protein domains, and entire proteins that are fused at the genetic level so the cell’s endogenously produced proteins of interest (light blue) will have the imparted fusion tag’s biochemical properties. Affinity reagents (green) are proteins, peptides, nucleic acids, and small chemical molecules that bind targets of interest with high specificity and affinity. There are many possible affinity reagents for each protein.