The work differs from the kind of DNA fingerprinting used to identify biological evidence left at a crime. It is extremely difficult—sometimes impossible—to extract conventional nuclear DNA markers from an old bone. The center has become skilled in extracting and analyzing a hardier but less-known source of DNA: that of the mitochondria that reside in our cells.Except for identical twins, each person's nuclear DNA is unique. But each of us has another set of DNA located outside the cell's nucleus and inside the mitochondria, the tiny organs that supply a cell with energy. We inherit mitochondrial DNA, known as mtDNA, directly from our mothers, and we share it with our siblings. It's not unique, but mtDNA is enough to narrow the search for a victim's family.Sansom spent almost an hour scrubbing and sanding the femur's surface before attempting extraction. Few of the bones here contain marrow, which dissolves in the first two or three years after death. F2775.1EC had spent some 20 years in a box inside a police warehouse, so DNA would have to come from the scant cellular material inside the bone's white scaffolding. She used a woodworker's dremel to cut a rectangular window in the thickened area of bone just below the femur's rounded head, where the thigh muscles once attached. Next she chilled, pulverized, and blended the sample inside a freezer mill loaded with sterilized ball bearings. Using an automated chemical process, she broke open the bone cells, released their genetic contents, and washed, concentrated, and purified the extract.