Biochemistry. In the article “Minimal and optimal mechanisms for GroE-mediated protein folding” by Anat Peres Ben-Zvi, Jean Chatellier, Alan R. Fersht, and Pierre Goloubinoff, which appeared in number 26, December 22, 1998, of Proc. Natl. Acad. Sci. USA (95, 15275–15280), the following correction should be noted. The legend of Fig. 4 should read as follows: “Effect of Mn²⁺ ions on GroEL/GroES-mediated refolding of mtMDH. The time-dependent reactivation of GroEL-bound mtMDH (4 μM GroEL, 0.3 μM mtMDH) was measured in the presence of 1 mM ATP and an ATP-regeneration system [equimolar (4 μM) GroES] in folding buffer as in Figs. 1–3, but with a near-limiting 6 mM concentration of divalent ions instead of 20 mM Mg²⁺. Open symbols: 6 mM Mg²⁺. Filled symbols: 4 mM Mg²⁺ and 2 mM Mn²⁺. A 6.25-fold excess of free GroEL (22 μM) was added (○, ▴) or not (□, ⧫) to the reaction 11 min after initiation of reactivation at 25°C with ATP.”