DNA inside energy-producing organelles called mitochondria is destroyed in a dad’s sperm shortly after it fertilizes an egg, researchers report online June 23 inScience. A protein called CPS-6 cuts apart the mitochondrial DNA in the male sperm so that the DNA can’t make the proteins that the mitochondria need to power the cell. Lingering paternal mitochondrial DNA might hurt developing embryos, the researchers say. “This is a very long-standing mystery in biology ,why in so many organisms, [only] the maternal mitochondria are inherited,” says Ding Xue, a geneticist at the University of Colorado Boulder who led the work.