The target enzyme of SDH inhibitors is succinate dehydrogenase (SDH, so-called complex II in the mitochondrial respiration chain), which is a functional part of the tricarboxylic cycle and linked to the mitochondrial electron transport chain (Keon et al., 1991). SDH consists of four subunits (A, B, C and D) and the binding site of the SDHIs (the ubiquinone binding site) is formed by the subunits B, C and D. Target site mutations conferring reduced sensitivity can develop in all three subunits.