In 1990, Dr. Mary-Claire King, a professor at the University of California, made one of the most important cancer discoveries in modern times when she demonstrated that breast cancer within families was passed on by a specific gene. Although King actually did not find the gene, her research overturned an inaccurate, prevailing theory about hereditary diseases, paving the way for a better understanding of how inherited genetic mutations cause cancer. Before King's discovery, scientists thought several genes interacted with certain environmental factors to cause breast cancer and other hereditary diseases, making it all but impossible for geneticists to identify who was likely to become ill. King, whose best friend died of kidney cancer in her early adolescence, was determined to improve the odds of survival. For 15 years, King studied DNA from women with family histories of breast cancer, looking for markers--genes always on the same chromosome as the cancer-causing gene. If the same marker pattern repeated in different DNA samples, King felt it might indicate a mutated gene was being passed on. King compared her work to "looking for an address in a strange town at night, with street lights every ten blocks." The team looked at 183 possible markers before they found a match, which led them to a region on the 17th chromosome that consistently was altered in women with cancer. They were unable to point to the actual gene, however. They could suggest only that it existed in that general area. When another researcher confirmed King's findings, an international race began to find and then clone "King's gene." Mark Skolnick, a scientist at the University of Utah and a longtime rival of King's, narrowed down the location to smaller and smaller regions of DNA, eventually locating the gene in 1994. Today, researchers have a much-improved understanding of how mutated genes cause hereditary breast cancer, which may soon lead to more effective treatment of the deadly disease.