SDHD (Succinate Dehydrogenase Complex Subunit D) is a protein that plays a crucial role in the electron transport chain of mitochondria, which is essential for energy production in cells. The SDHD gene provides instructions for making one of the four subunits of the succinate dehydrogenase (SDH) complex, also known as complex II or succinate:quinone oxidoreductase. This complex is unique because it participates in both the citric acid cycle and the electron transport chain.