Cells from an SDH-linked paraganglioma tumor have a defect in Complex II (succinate dehydrogenase) of the electron transport chain. SDH (succinate dehydrogenase) is another name for Complex II, and this enzyme is responsible for catalyzing one of the reactions in the electron transport chain. SDH-linked paraganglioma is a rare tumor syndrome that is caused by mutations in the genes that encode for this enzyme, and the defect in SDH leads to an accumulation of succinate, which promotes the growth of tumors.