Abstract: The present invention provides methods for introducing functional peptides into organelles. Additionally, the present invention provides a method for correcting a phenotypic deficiency in a mammal that results from a mutation in the mammal's mitochondrial DNA (mtDNA). The present invention further provides a method for treating a mitochondrial disorder in a subject in need of treatment therefor. Also provided is an expression vector that is useful for introducing a functional peptide encoded by an mtDNA sequence into a mitochondrion. The present invention also provides eukaryotic cells transformed by expression vectors that are useful for introducing functional peptides into organelles. Finally, the present invention provides a pharmaceutical composition comprising a non-nuclear nucleic acid sequence encoding a peptide for introduction into an organelle, a nucleic acid sequence encoding an organelle-targeting signal, and a pharmaceutically-acceptable carrier.