DNA assembly allows scientists to engineer cells to produce medications and chemicals, convert plant material into biofuel, and develop novel bio-based materials. Scientists pursuing these goals regularly seek to combine genes encoding a series of enzymes (biological catalysts) that perform the necessary reactions. This requires designing DNA parts – genes for multiple enzymes and shorter DNA sequences that regulate expression of those genes – to form DNA assemblies. The DNA assemblies are inserted into host cells, where they are expressed to produce the enzymes that carry out targeted reactions. While many of the steps involved in this type of genetic engineering have become easier with technological advances in the past several decades, the design of complex DNA assemblies and efficient testing for function remains particularly challenging. Nevertheless, assemblies of increasing complexity and variability are needed for research vital to human health and the health of the planet.