SNAP-tag is a small, engineered self-labeling protein. It is derived from human O6-alkylguanine-DNA-alkyltransferase. 

SNAP -Tag reacts specifically and covalently with O6-benzylguanine derivatives such as fluorescent dyes conjugated to chloropyrimidine or guanine. This allows for stable attachment of biotin, fluorophores, and other functional molecules to the tagged protein.