BG azide is an alkyne-reactive building block for preparing SNAP substrates from the alkyne-reactive forms of labels or biological targets. This BG azide building block facilities the development of various SNAP-tag substrates for labeling SNAP-tag fusion proteins for a wide range of biological applications via the well-known click chemistry. AAT Bioquest offers a broad range of alkyne-reactive fluorescent probes and other Tag probes that can be readily coupled to BG azide. The SNAP protein labeling system enables the specific, covalent attachment of virtually any molecule to a protein of interest. The SNAP-tag is a protein based on human O6-alkylguanine-DNA-alkyltransferase (hAGT). SNAP-tag substrates are fluorophores, biotin or beads conjugated to guanine or chloropyrimidine leaving groups via a benzyl linker. In the labeling reaction, the substituted benzyl group of the substrate is covalently attached to the SNAP-tag.