Mitochondrial disorders are a group of heterogeneous diseases associated with abnormalities of the
oxidative phosphorylation (OXPHOS), the most important source of energy for the cell. The number of
mitochondrial syndromes and of identified causative genes is constantly increasing. Taken as a whole they are
among the most frequent genetic diseases in humans at any age. The respiratory chain is the only metabolic
pathway under double genome control and molecular genetics of these disorders is complicated by the
existence of strict interactions between mitochondrial DNA and nuclear DNA. In childhood and infancy, clinical
presentation differs from mitochondrial disorders with adult onset. The phenotypes are much more severe,
often involving brain, frequently presenting as multisystemic disorders and seldom as isolated myopathy.
Mutations in nDNA are more frequent than in adulthood.
The major phenotypes presenting in infancy are here correlated with genetic defects and biochemical data with
the aim to facilitate diagnosis work-up.