Membrane-anchoring subunit of succinate dehydrogenase (SDH) that is involved in complex II of the mitochondrial electron transport chain and is responsible for transferring electrons from succinate to ubiquinone (coenzyme Q). Carbohydrate metabolism; tricarboxylic acid cycle. Forms part of complex II containing four subunits: a flavoprotein (FP), an iron-sulfur protein (IP) and a cytochrome b composed of a large and a small subunit. Increased sensitivity to phosphine. Belongs to the CybS family.