The SNAP tag is 182-amino acid long, 12.4 kDa protein epitope derived from the mammalian alkylguanine-DNA alkyltransferase, also known as AGT or MGMT. A key feature of the SNAP tag is its ability to become irreversibly labeled by benzylguanine (BG) derivatives, such as in those in the form of fluorophores. This allows for the quantitative labeling of fusion proteins. In addition, antibodies are also available for the immunological investigations of SNAP-tagged proteins.