Nature 391, 96–99 (1998); doi:10.1038/34214

Recently, it has come to our attention that in Fig. 1a of this Letter, lanes 1 and 5 appear to be duplicated and lanes 6 and 10 appear to be duplicated. It is unclear how this happened. We have repeated the experiment (see Fig. 1 of this Corrigendum) and the results were as described in the original Letter. The Supplementary Information to this Corrigendum contains the source data used to generate the corrected Fig. 1. Our conclusions are unaffected.

Figure 1
figure 1

This is the corrected Fig. 1a of the original Letter.

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