||Succinate dehydrogenase (SDH, also known as complex II) is a protein complex located in the inner mitochondrial membrane. SDH has dual roles as a part of both the citric acid cycle and the electron transport chain required for aerobic respiration in eukaryotes. In the past two decades, the structure of SDH has been extensively studied. The tetrameric complex consists of SDHA, SDHB, SDHC and SDHD (Sdh1, Sdh2, Sdh3 and Sdh4 in yeast) and contains five cofactors: one flavin adenine dinucleotide, three Fe- S clusters and one heme b. Mutations in SDH subunit genes are associated with various human diseases. Recently, interest has been refocused on SDH by discovery of SDH assembly factors, SDHAF1 and SDHAF2. However, the mechanism for SDH assembly was still poorly understood. The fact that SDH consists of four subunits and five cofactors supports the idea that maturation of SDH would require several assembly factors, as is the case for other electron transport chain complex I, III and IV. SDHdeficient gastrointestinal stromal tumors and neuroblastomas associated with SDH deficiency without mutations in genes encoding SDH subunits or two aforementioned assembly factors also suggest additional unknown SDH assembly factors. In this study, we focus on understanding the mechanism for maturation of the Fe- S cluster subunit of SDH, Sdh2. We discovered a novel SDH assembly factor, Sdh7, and characterized its function. We also revealed the molecular function of Sdh6, a yeast ortholog of human SDHAF1, whose mutations were shown to result in SDH deficiency iv through an unknown mechanism. We demonstrate that Sdh6 and Sdh7 impart protection for the Fe-S cluster subunit of SDH against reactive oxygen species under oxidative stress conditions during SDH assembly. We also propose that a sequence variant of SDHAF3, a yeast ortholog of Sdh7, could be a pathogenic allele that enhances predisposition to endocrine-related tumors. Lastly, we elucidate the function of Nfu1 in Fe-S cluster delivery to target proteins including Sdh2.