Disulfide-bond formation in the purified human Bach2 POZ domain in solution. (a) Analysis of purified wild-type Bach2 POZ-domain protein by SDS–PAGE. The sample in lane 1 contained 20 mM DTT and the samples in lanes 2–8 were treated with various concentrations of hydrogen peroxide for 1 h at room temperature and electrophoresed under nonreducing conditions. (b) Analysis of purified C20S Bach2 POZ-domain protein by SDS–PAGE. Samples were treated as described in (a). (c) Purified wild-type Bach2 POZ-domain protein, together with the C20S, C125S and C20S/C125S mutants, was analysed by SDS–PAGE. Reduced samples were treated with 5% β-mercaptoethanol (βMe). Oxidized samples were incubated with 1 mM hydrogen peroxide for 1 h at room temperature.