On Oct. 11, 1985, scientists in Canada and Massachusetts reported discovering the first genetic marker for the widespread and devastating hereditary disease cystic fibrosis (CF) . The marker is a telltale segment of deoxyribonucleic acid, or DNA, the substance of the genes of all living things. The particular DNA segment discovered in cystic fibrosis patients is a genetic variant that appeared to be inherited with the gene that is faulty in cystic fibrosis.

This distinguishing variant, found in cystic fibrosis patients and their families, is not the faulty gene itself and probably does no harm. Its importance is that it lies close enough to the faulty gene on one of the 23 pairs of human chromosomes so that the two are almost always inherited together.