A RNA polymerase III promoter drives expression of a transcription unit composed of a non-coding leader (NCL) sequence (from the RNaseMRP:RNA gene), a translation initiation sequence and the EGFP open reading frame (ORF). The ORF has been engineered for mitochondrial translation and a UGA cytosolic stop codon (that encodes the Trp residue in mitochondria) has been introduced at codon 58. The presence of the NCL sequence targets the RNA to the mitochondrion and the presence of the early cytosolic stop codon is designed to allow expression of the full-length EGFP ORF in the mitochondrion but not in the cytosol.