NARP: Neuropathy, Ataxia and Retinitis Pigmentosa. NARP is a disease featuring weakness of the muscles near the trunk, ataxia (wobbliness), retinal disease, seizures and developmental delay. NARP is a mitochondrial disease in that it is due to a mutation (change) in mitochondrial DNA (mtDNA), the DNA of the mitochondrion, a structure situated in the cytoplasm of the cell rather than in the nucleus where all the other chromosomes are located. All mtDNA is inherited from the mother. There are 2 to 10 copies of the mtDNA genome in each mitochondrion. mtDNA is a double-stranded, circular molecule. It is very small relative to the chromosomes in the nucleus and so contains only a limited number of genes. It is specialized in the information it carries and encodes a number of the subunits in the mitochondrial respiratory-chain complex that the cell needs to respire. Mutations (changes) in mtDNA can cause disease. The mutations often impair the function of oxidative-phosphorylation enzymes in the respiratory chain. This is especially manifest in tissues with a high energy expenditure such as brain and muscle.