Strategy for creating rps4 -null cells. As a starting material, LpCSfo cells in which pCoxIV (MTS)-Sfo I is expressed under the tetracycline-minus (-Tet) condition, were prepared. Subsequently, the mutant rps4 gene (Mut-mtDNA for homologous recombination), in which the upstream Sfo I-site and a 5'-half of rps4 coding region were deleted, was introduced into LpCSfo cells to obtain heteroplasmic transformants (LpCSfoHR cells) with mitochondria consisting of the Mut-mtDNA and wild-type mtDNA (Wt-mtDNA). Coupled with removal of Tet from growth medium, the fusion protein MTS-Sfo I synthesized in the cytoplasm is exclusively transferred into mitochondria of LpCSfoHR cells and selectively digests Wt-mtDNA but not Mut-mtDNA. Since the digested Wt-mtDNA is not duplicated, the Mut-mtDNA becomes dominant during the course of growth under the -Tet condition, thus eventually giving rps4-null cells.