Polybrominated diphenyl ethers and polybrominated bipyrroles are found naturally throughout the marine food chain. These compounds are of interest because they persist in the environment and are potentially toxic to humans. Until now, no one knew exactly how they were produced. To find the responsible biosynthetic pathway, Bradley S. Moore and coworkers at the University of California, San Diego, cultured two species of marine bacteria known to produce such compounds and sequenced their genomes (Nat. Chem. Biol. 2014, DOI: 10.1038/nchembio.1564). They then identified candidate gene clusters and engineered them into Escherichia coli bacteria. The E. coli, which does not normally produce polybrominated compounds, successfully biosynthesized them, confirming that the genes were correct. The researchers found similar gene clusters in other marine bacteria. They also identified enzymes that produce monomeric bromophenol and bromopyrrole building blocks, as well as a cytochrome P450 enzyme that couples those building blocks into homo- and heterodimers. As synthetic polybrominated compounds continue to be phased out as flame retardants, the UC San Diego researchers suspect there will be increased interest in knowing more about natural polybrominated compounds.