Mitochondria, best known for their role as cellular power plants, perform numerous vital tasks in the cell. During cell respiration, reactive oxygen species can be formed in mitochondria. If these are present in excess, their high reactivity leads to irreparable damage to important cellular components. This so-called oxidative stress is assumed to play a causal role in many diseases and in ageing processes. In low concentrations, however, reactive oxygen species can also act as important second messengers in the cell. Here, specific, so-called redox-active thiols in distinct proteins are modified. This type of oxidative modification is reversible and, like a nano-switch, can regulate the function of a protein.