BioID2 corresponds to a promiscuous biotin ligase derived from the Aquifex aeolicus birA gene (UniProt/ TrEMBL:O66837 ). It is fundamentally comparable to BirA(R118G) (aka BioID) in its ability to promiscuously biotinylate proximate proteins but is substantially smaller and requires considerably less biotin (PMID:26912792). BioID2 therefore enables more selective targeting of fusion proteins and exhibits enhanced labeling of proximate proteins compared to BirA(R118G).