Comparing the physical characteristics of different hair strands under a microscope generally can do no better than identify large groups of people, such as adult white males, who could be the source of a particular hair sample. And conventional DNA testing will work only on a hair with an intact root. Mitochondrial DNA testing uses DNA extracted from the energy-producing material found in the portion of a cell that surrounds the nucleus. Although not as precise as nuclear DNA testing, mitochondrial DNA testing works on materials that contain very old, very degraded or very small amounts of DNA, where nuclear DNA testing is ineffective. Since its introduction into the courtroom about 2 years ago, mitochondrial DNA testing has been used as evidence in at least seven criminal cases in six States, according to the FBI. However, use of the technique is not without its critics, some of whom claim such evidence is used more for its dramatic effect than for its probative value.