Mitochondrial functions are crucial for organism biogenesis development and survival. Their biogenesis and maintenance involve extremely complex processes that require diverse and well-coordinated regulatory pathways. Most of the mitochondrial disorders are due to dysfunction of the respiratory chain. Both mitochondrial and nuclear genomes encode respiratory chain components. Mutations in either genome can cause respiratory chain dysfunction. Coenzyme Q (CoQ) is an electron carrier in the respiratory chain and its deficiency impairs oxidative phosphorylation and causes clinically heterogeneous mitochondrial diseases. Eight genes have been identified in the human genome required for CoQ biosynthesis, although the existence of another genes may be involved. We hypothesize that mutations in any of these genes could cause primary CoQ deficiency and mutations in different genes may explain the clinical heterogeneity of this syndrome. We propose in this project the integration of basic and applied research in a multidisciplinary effort to coordinate the expertise of different European research groups and enterprises. We propose the development of a reliable and easily applicable molecular procedure to reach an unequivocally diagnostic of the genetic origin of mitochondrial diseases that course with primary CoQ deficiency. Research activities are grouped in four work packages (WPs) that will incorporate the scientific objectives. A group of another WPs will include innovation activities such as protection and exploitation of products. Demonstration activities will be carried out in a WP dedicated to the standardization and testing of products. Management will be highly considered by the establishment of internal controls both in science and finances. A special attention will be dedicated to training and mobility activities. Also, activities related to gender and ethical aspects are considered in two WPs.