Oxidation reversibly activates CIC-2, and a four-cysteine cluster was identified as being potentially important for oxidant-dependent activation. I assessed the role of these cysteines in activation following mutagenesis. Unlike C256A, mutagenesis of these cysteines to serines did not alter cell surface expression. This mutant lacked activation by oxidation as assessed using an iodide efflux assay. Therefore, these cysteines are likely involved with oxidation-dependent gating.CIC-2 is a voltage-gated chloride channel that is important in neuronal chloride homeostasis. Mutations of CIC-2 in humans cause epilepsy. Channel opening is controlled by two gates.Zinc inhibits CIC-2 by closing the slow gate. Cysteine 256 has been implicated in metal binding because its mutation to alanine abolishes metal-evoked inhibition. However, I found using affinity chromatography that C256 was not required for metal binding. Rather, the mutation altered protein abundance at the cell surface. Therefore, loss of metal-sensitive inhibition likely reflects decreased cell surface expression.