Nat. Struct. Mol. Biol. 13, 668–670 (2006).

In the version of this article initially published, reference 3 is incorrect. The correct reference should be as follows: Prakash, S., Tian, L., Ratliff, K.S., Lehotzky, R.E. & Matouschek, A. Nat. Struct. Mol. Biol. 11, 830–837 (2004).