BirA(R118G) corresponds to a promiscuous biotin ligase derived by mutation of the Escherichia coli birA gene (EcoGene:EG101230); it contains a R118G amino acid substitution relative to the wild-type protein (PMID:15459338). The mutation results in promiscuous biotin ligase activity that shows proximity dependence (PMID:15459338, PMID:22412018); the labeling radius for BirA(R118G) is approximately 10nm in vivo (PMID:24927568). BirA(R118G) can thus be used as a proximity labeling tool for proteomic analysis (this technique is often referred to as BioID, for 'proximity-dependent biotin identification'). The major disadvantage of BirA(R118G) is its slow kinetics; labeling with biotin for several hours is required to produce sufficient biotinylated material for analysis. The exact sequence of the BirA(R118G) biotin ligase may vary depending on the particular transgene or modified endogenous locus being used; the coding sequence may have been codon optimized for use in different species, and amino acid residue A146 may have been deleted to suppress dimerization. For a comparison of BirA(R118G) with other protein proximity labeling tools, see PMID:26355100, PMID:29125959, PMID:30774936, PMID:27667171, FBrf0240093.