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Acknowledgements
This work was supported by NIH grant R01GM065865 and the Howard Hughes
Medical Institute. We thank Y. Shen and C. Dumelin for MS assistance, and D. Gorin
for discussions.
Author contributions
Y.C., A.S.K., J.B.S. and D.R.L. designed the research, analysed the data and co-wrote the
manuscript, Y.C., A.S.K. and J.B.S. performed the experiments.
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Additional information
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