Nature 469, 362–367 (2011)

On page 2 of the PDF and print version of this Article, column one, under the heading ‘Genetic basis of functional heterogeneity’, line 9 should refer to Fig. 1e instead of Fig. 2e. Similarly, in column two of the same section, lines 7 and 9 should ref to Fig. 1f and 1g, respectively, instead of Fig. 2f and 2g. These have now been corrected in the HTML version of the manuscript.