Nature Chemistry 5, 582–589 (2013); published online 9 June 2013; corrected after print 21 January 2016.

In the version of this Article originally published, the concentration of trypsin and chymotrypsin in the Methods section 'In vivo imaging of enzyme activity' should have read “0.4 mg ml−1 in 50 mM phosphate buffer pH 7”. This error does not affect the conclusions and has been corrected in the online versions of the Article.