Abstract: The present invention provides a method for introducing a functional peptide encoded by a plant or protist nucleic acid sequence into a mitochondrion of a mammalian cell, and a pharmaceutical composition comprising the nucleic acid sequence. The present invention also provides a method for correcting a phenotypic deficiency in a mammal resulting from a mutation in a mitochondrial peptide. Additionally, the present invention is directed to a method for treating a mitochondrial disorder in a subject in need of treatment therefor. The present invention further provides expression vectors for use in introducing a functional peptide encoded by a plant or protist (including algal) nucleic acid sequence into a mitochondrion of a mammal, as well as mammalian cells transformed by the expression vectors. Also provided are clonal cell strains comprising the transformed mammalian cells. Finally, the present invention is directed to a method for introducing a functional peptide into a mitochondrion.