Nature 481, 209–213 (2012); doi:10.1038/nature10697

The legend to Fig. 1d of this Letter incorrectly referred to “a POPS lipid's amino NH3+ group (blue)”. It should read “a POPC lipid's choline N(CH3)3+ group (blue)”, where POPC is 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine. The original legend to Fig. 1d has been corrected.