Owing to the peculiar properties of their DNA-binding specificity, transcription activator-like effectors (TALEs) from the plant pathogen Xanthomonas can be engineered to bind virtually any DNA sequence. Jaenisch and colleagues exploit this property to drive targeted modifications in both human embryonic stem cells and induced pluripotent stem cells (Nat. Biotechnol. 29, 731–734; 2011), in which genetic engineering by homologous recombination is inefficient.

TALEs bind DNA through 34-amino-acid-long repeats, in which two amino acids per repeat are hypervariable to recognise a specific base. Rearrangements of TALE repeats allow the generation of novel DNA-binding specificities. When TALE domains are coupled to an endonuclease, TALENs can target endogenous genes, creating deletions or fusion proteins to monitor expression. The authors used this system to target five different genomic sites in pluripotent stem cells and generated clones expressing fusion proteins of both the protein OCT4, which is expressed in pluripotent cells, and the protein PITX3, which is only induced following differentiation.

In a separate study, Jaenisch and colleagues used zinc-finger nucleases, which have previously been developed for genetic engineering in pluripotent stem cells, to create isogenic human pluripotent stem cells differing only in the presence of specific α-synuclein variants that had been linked to early onset of Parkinson's disease (Cell 146, 318–331; 2011). Using zinc-finger technology, however, can be laborious as extensive optimization is required to construct nucleases that would target unique sites of the genome. TALENs, with their defined combinatorial properties, could become the future of genetic engineering in stem cells.