The question is approached by isolating a large number of independent mutations in the same gene. Many mutants are obtained ,each of which has suffered an individual mutational event . Then the site of each mutation is determined. Most mutations will lie at different sites, but some will lie at the same position. Two independently isolated mutations at the same site may constitute exactly the same change in DNA (in which case the same mutational event has happened on more than one occasion), or they may constitute different changes (three may constitute different changes (three different point mutations are possible at each base pair.)