The mitochondrial respiratory chain (MRC) enzymes associate in supercomplexes (SC). This structural interdependency may explain why defects in a single component often produce combined enzyme deficiencies in patients. A point in case is the alleged destabilization of complex I in the absence of complex III. To clarify the structural and functional relationships between complexes, we have used comprehensive proteomic, functional and biogenetical approaches to analyze a MT-CYB-deficient human cell line.