Nature 439, 871–874 (2006)

It has been drawn to our attention that in this Letter some of the lanes in Figures 2a, 2c, 4a and Supplementary Fig. S1 were inadvertently missing. A description of how the figures were made is provided in Supplementary Information to this Corrigendum, together with the corrected figure panels. Our results and conclusions are unaffected by these oversights.