MDH2 and pheochromocytoma: Dominant defects associated with tumor development have been reported in three Krebs cycle enzymes: isocitrate dehydrogenase 1 (IDH1), SDH, and fumarate hydratase (FH),19, 20, 21 and MDH2 was also recently identified as a pheochromocytoma and paraganglioma susceptibility gene.4