results support the sequential mechanism and provide the rate constants shown in Figure 2. However, many other aspects of the maturation mechanism remain obscure, such as the steric and catalytic roles of neighboring residues, the means by which mutations can improve folding efficiency, and the dependence of oxidation rate on the oxygen concentration and the protein sequence. One predicted consequence of oxidation by O2 is that hydrogen peroxide, H2O2, is presumably released in 1:1 stoichiometry with mature GFP. This byproduct might explain occasions when high-level expression of GFP can be deleterious. Perhaps catalase could be useful in such cases. Some difficult GFPs seem to express mostreadilywhentargetedtoperoxisomesandmitochondria (28; R Rizzuto, T Pozzan, personal communication). Is it a coincidence that these organelles are the best at coping with reactive oxygen species?