BirA(opt) corresponds to a promiscuous biotin ligase derived from the Escherichia coli birA gene (EcoGene:EG101230). It is a humanized version of birA, with most of the rare codons replaced by codons that are more frequently used in human cells, resulting in better expression in mammalian cells and improved efficiency of biotinylation in vivo. (PMID:15707685, PMID:28098257.)