Nature 418, 617–620 (2002).

For this Letter the disclosure form for the declaration of competing financial interests was incorrectly filled out because of a misunderstanding. The statement should have read: ‘The authors declare competing financial interests: details accompany the paper on Nature's website (http://www.nature.com/nature).’ The details on the website should have read: ‘O.L.K. and N.D. have a personal financial interest in Nion, R&D.’. We (the authors) had no intention of misrepresenting the origin of the work.