Nat. Rev. Dis. Primers article number: 15021; doi:10.1038/nrdp.2015.21; published online 27 August 2015

In the version of the article originally published, details of the source of the images in Figure 4a and 4b were omitted. This has now been corrected: Images in a and b are courtesy of H. P. Singh, Children's Hospital Los Angeles, California, USA.