Human mitochondrial DNA, the 25th chromosome, is a 16 569 base pair long 
circular molecule, that encoders a variety of genes for the translational 
machinery of the mitochondrion, as well as 13 structural proteins, that are all 
subunits of the respiratory chain (RC). A variety of alterations of 
mitochondrial DNA (mtDNA) are now functionally and genetically linked to human 
disease, including encephalomyopathies, Leber's hereditary optic neuropathy, 
diabetes mellitus, some neurogenerative diseases and even ageing. The 
mitochondrial encephalomyopathies are a clinically, biochemically and 
genetically heterogeneous group of disorders. Alterations of mtDNA include point 
mutations, encephalomyopathies, as well as in the other human diseases, are 
reviewd.