The present invention relates to a method for producing in vitro a homo- or heteroplasmic cell or cell population from a donor and a recipient cell: providing an enucleated recipient cell comprising mitochondrial DNA (mtDNA), fusing said recipient cell with a donor cell or introducing a nucleus of a donor cell into the recipient cell, and cultivating the fused recipient cell, characterized in that the mitochondrial DNA (mtDNA) of the recipient cell is at least in part mildly deleterious.
