Nature Chemistry 2, 138–145 (2010); published online 20 December 2009; corrected after print 13 March 2012 .

The authors wish to add the following to the Acknowledgements section of this Article:

We gratefully acknowledge the use of the UCSB Laboratory for Stem Cell Biology and Engineering and funding from the California Institute for Regenerative Medicine.

The online versions of the Article have been amended accordingly.