Matthew W. Powner*a, John D. Sutherlandb, Jack W. Szostaka Howard Hughes Medical Institute, and Department
of Molecular Biology and Center for Computational and Integrative
Biology, Massachusetts General Hospital, 185 Cambridge Street, Boston, Massachusetts 02114, USA Fax: +1(617)6433328; e-Mail: powner@molbio.mgh.harvard.edu; MRC Laboratory of Molecular Biology, Hills Road, Cambridge, CB2 0QH, UK
