Nature Reviews Immunology 9, 556–567 (2009); published online 3 July 2009; corrected after print 7 August 2009

In the version of this article initially published, reference 22 was incorrectly stated to be the first report to directly compare Il17a−/− and Il17f−/− mice and to show that these cytokines have markedly different functions in vivo. In fact, this was first reported by reference 19: Yang, X. O. et al. Regulation of inflammatory responses by IL-17F. J. Exp. Med. 205, 1063–1075 (2008)