Site-specific labeling of cellular proteins with chemical probes is a powerful tool for studying protein functions in living cells. The SNAP-tag is a modified form of the DNA repair enzyme, human O6-alkylguanine-DNA-alkyltransferase (AGT) and undergoes a self-labeling reaction to form a covalent bond with O6-benzylguanine (BG) derivatives. BG-SiR conjugate is a fluorescent SNAP substrate that can be used to label SNAP tag-fused target proteins. In the labeling reaction, the substituted benzyl group of the substrate is covalently attached to the SNAP-tag. SNAP-SiR substrate enables the specific, covalent attachment of SiR fluorophore to a protein of interest. SiR has a similar fluorescence spectrum to Cy5 or Alexa Fluor 647. The labeled proteins can be imaged with Cy5 filter set.