Nat. Struct. Mol. Biol. 24, 370–378 (2017); published online 20 February 2017; corrected after print 27 April 2017

In the version of this article initially published, funding information for B.C. and C.S.P. was missing NIH grant S10 OD019994-01. In addition, there was an incorrect comma in the introduction (after “glycoprotein” in the sentence “Upon binding to the primary cellular receptor, CD4, the external gp120 Env glycoprotein undergoes major conformational changes...”) that has been removed. The errors have been corrected in the HTML and PDF versions of the article.