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During the roughly two billion years of endosymbiotic co-evolution, many ancestral genes in proto-mitochondria have transferred to the nucleus of eukaryotic cells. In this issue, a study finds that transmission of mitochondrial DNA to the nuclear genome is an ongoing process. It occurs frequently in human cancer cells, reflecting frequent contact between the two genomes in human somatic cells despite multiple physical barriers. The cover represents the evolution of eukaryotic cells through endosymbiosis of mitochondria. (This intaglio is a part of a complete print, Symbiogenesis [2009] by Eveline Kolijn, http://evelinekolijn.com. [For details, see Ju et al., pp. 814–824.])