Nature 517, 501–504 (2015); doi:10.1038/nature13868

Owing to a production error, in Fig. 1b of this Letter, the key should have shown that the black bars corresponded to ‘WT’ and the red bars to ‘γ1’, instead of the other way round. In addition, in Fig. 1c the ‘WT’ label was missing from the mouse on the left, and the ‘γ1’ label should have applied to the mouse on the right. These errors have been corrected in the online versions of the manuscript.