The mitochondrial DNA of plant and animal cells is a transcriptionally active genome that traces its origins to a symbiotic infection of eucaryotic cells by bacterial progenitors. As prescribed by the Serial Endosymbiosis Theory, symbiotic organelles have gradually transferred their genes to the eucaryotic genome, producing a functional interaction of nuclear and mitochondrial genes in organelle function. We report here a recent remarkable transposition of 7.9 kb of a typically 17.0-kb mitochondrial genome to a specific nuclear chromosomal position in the domestic cat. The intergrated segment has subsequently become amplified 38-76 times and now occurs as a tandem repeat macrosatellite with multiple-length alleles resolved by pulse-field gel electrophoresis (PFGE) segregating in cat populations. Sequence determination of the nuclear mitochondrial DNA segment, Numt, revealed a d(CA)-rich 8-bp motif [ACACACGT] repeated imperfectly five times at the deletion junction that is a likely target for recombination. The extent and pattern of sequence divergence of Numt genes from the cytoplasmic mtDNA homologues plus the occurrence of Numt in other species of the family Felidae allowed an estimate for the origins of Numt at 1.8-2.0 million years ago in an ancestor of four modern species in the genus Felis. Numt genes do not function in cats; rather, the locus combines properties of nuclear minisatellites and pseudogenes. These observations provide an empirical glimpse of historic genomic events that may parallel the accommodation of organelles in eucaryotes.