Cell-free transcription–translation: engineering biology from the nanometer to the millimeter scale

Cell-free transcription–translation (TXTL) has become a highly versatile technology to construct, characterize and interrogate genetically programmed biomolecular systems implemented outside living organisms. By recapitulating gene expression in vitro, TXTL offers unparalleled flexibility to take apart, engineer and analyze quantitatively the effects of chemical, physical and genetic contexts on the function of biochemical systems, from simple regulatory elements to millimeter-scale pattern formation. Here, we review the capabilities of the current cell-free platforms for executing DNA programs in vitro. We describe the recent advances in programming using cell-free expression, a multidisciplinary playground that has enabled a myriad of novel applications in synthetic biology, biotechnology, and biological physics. Finally, we discuss the challenges and perspectives in the research area of TXTL-based constructive biology.