CHEMISTRY, BIOPHYSICS. For the article “Interplay among side chain sequence, backbone composition, and residue rigidification in polypeptide folding and assembly,” by W. Seth Horne, Joshua L. Price, and Samuel H. Gellman, which appeared in issue 27, July 8, 2008, of Proc Natl Acad Sci USA (105:9151–9156; first published June 27, 2008; 10.1073/pnas.0801135105), the authors note that the circular dichroism (CD) and analytical ultracentrifugation (AU) data reported in Fig. 3 and Table 1 for oligomer 8 were inadvertently obtained for a point mutant in which β³-hIle₁₂ was substituted with α-Ile. The authors have subsequently prepared and characterized an authentic sample of α/β-peptide 8. The authors state: “The folding and self-assembly of authentic 8 is qualitatively similar to that of the point mutant, although the quaternary structural stability of 8 is somewhat lower. Authentic 8 shows greater quaternary structural stability than does 5, and this improvement was the most important feature of 8 with respect to the main thesis of the article. Therefore, this error does not affect the conclusions of the article.” The corrected figure and its legend, and the corrected table, appear below.