Transmembrane protein 126B is a protein that in humans is encoded by the TMEM126B gene. TMEM126B is a mitochondrial transmembrane protein which is a component of the mitochondrial complex I assembly complex. The TMEM126B gene is conserved in mammals. The encoded protein serves as an assembly factor that is required for formation of the membrane arm of the complex. It interacts with NADH dehydrogenase [ubiquinone] 1 alpha subcomplex assembly factor 13. Naturally occurring mutations in this gene are associated with isolated complex I deficiency. A pseudogene of this gene has been defined on chromosome 9.