Mitochondria and chloroplasts are descended from free-living bacteria. The endosymbiosis theory explains why organelle genomes are significantly smaller than that of proteobacteria and cyanobacteria. We study endosymbiotic gene transfer, specifically the current transfer of mitochondrial and plastid DNA fragments into eukaryotic nuclear genomes which giving rise to nuclear copies of mitochondrial DNA (numts) and nuclear copies of plastid DNA (nupts).