The GST epitope tag is derived from glutathione S-transferase, a protein with a high affinity for glutathione. GST-glutathione binding is the principle behind the GST pull-down assay, which uses glutathione-coated beads to purify GST-tagged proteins. One limitation is that GST is larger than other epitope tags, with an amino acid length of 220 and a mass of roughly 26 kDa. In addition to pull-down, antibodies may also be useful in the detection of GST-tagged proteins.