Linkage testing is done when the specific location is unknown. This is known as genetic linkage testing. The aim of this testing is to isolate the gene for this disease. This test is done for knowing that if they have the mutant gene which is responsible for causing this disease. There is always 50% negative and positive chance during genetic mapping testing. Huntington disease affects the brain and central nervous system. If the gene is found to contain thirty nine or more repeats, the person will develop this disease at some time. Thirty five or fewer repeats, the person will not develop this disease.