NEWS & VIEWS RESEARCH To determine which changes are beneficial us who we are. The authors’ work provides an 3. Urnov, F. D., Rebar, E. J., Holmes, M. C., Zhang, H. S. to the cell and which detrimental, Findlay excellent case in point to support the words & Gregory, P. D. Nature Rev. Genet. 11, 636–646 11 (2010). and colleagues used deep sequencing, which of geneticist Sydney Brenner : “Progress 4. Joung, J. K. & Sander, J. D. Nature Rev. Mol. Cell Biol. reads every copy of every gene in every cell in science results from new technologies, 14, 49–55 (2013). 5. Ran, F. A. et al. Nature Protocols 8, 2281–2308 of a population. Immediately after editing, new discoveries and new ideas, probably in (2013). the cells are a kaleidoscope of genetic diver- that order.” ■ 6. Urnov, F. D. et al. Nature 435, 646–651 (2005). sity. Edited cells account for only 1–3% of the 7. Goldberg, A. D. et al. Cell 140, 678–691 (2010). total cell population (lower than seen in other Fyodor D. Urnov is at Sangamo BioSciences 8. Doyon, J. B. et al. Nature Cell Biol. 13, 331–337 2 (2011). studies ), but this is not a real problem because Inc., Richmond, California 94804, USA. 9. Sexton, A. et al. Genes Dev. http://dx.doi. deep sequencing can identify even very rare e-mail:
[email protected] org/10.1101/gad.246819.114 (2014). DNA sequences. 10. Braberg, H. et al. Cell 154, 775–788 (2013). 1. Findlay, G. M., Boyle, E. A., Hause, R. J., Klein, J.