SpyTag and SpyCatcher form an interacting peptide-protein pair; the SpyTag peptide rapidly and spontaneously forms a covalent isopeptide amide bond with its specific binding partner, the SpyCatcher protein, upon mixing. SpyTag and SpyCatcher are artificial protein sequences engineered from the second immunoglobulin-like collagen adhesin domain (CnaB2) of the Streptococcus pyrogenes fibronectin binding protein (FbaB) gene; SpyTag is a small peptide based on the C-terminal β-strand of the CnaB2 domain, while SpyCatcher is a larger protein based on the remainder of the CnaB2 domain (both SpyTag and SpyCatcher contain additional mutations/modifications relative to the FbaB sequence, to optimize the rapid formation of the covalent bond between them). SpyTag and SpyCatcher can thus be used to bring together two tagged proteins of interest in vivo (PMID:22366317).