A field of biology that draws on principles of engineering to create new biological parts, devices, or systems, and to redesign or improve natural systems. Underlying synthetic biology is the capability to make DNA polymers with specific base sequences that behave as genes. These synthetic polymers consist of various small DNA sequences that are ‘stitched’ together by homologous recombination to yield functional genes with the required properties, e.g. multigene transcription units with promoters, ribosome-binding sites, and terminators. The corresponding proteins with desired properties can then be assembled when the engineered gene is matched with the necessary molecular apparatus. A common strategy is to introduce the engineered genes into a modified bacterium, such as E. coli. Entirely synthetic cells, e.g. based on protocells, are another option. Synthetic biology is becoming a vast new biotechnology sector, with a host of applications using cellular ‘factories’ designed to produce new materials, drugs, and vaccines, harness light energy via photosynthesis, degrade toxic chemicals, act as biosensors, or even function as biological computers, among others. Much interest centres on the potential of DNA to permanently store digital data. Files in digital format, including images, text, and music, comprise bits of information in the form of 1s and 0s. Pairs of this binary code can be assigned to one of the four bases that constitute the nucleotides of the DNA strands—A, C, G, and T. A DNA molecule is then assembled whose bases correspond to the sequence of digits in the data file. Subsequently the information can be retrieved by ‘reading’ the coding strand of the relevant DNA molecule by DNA sequencing. DNA is a high-density storage medium, is very durable if stored correctly, and can easily be replicated using existing technology such as PCR. At present the main obstacle is the slow speed of synthesizing and reading the DNA compared with, say, a hard drive.