Solidifying their hypothesis of the Cohens' common ancestor, they found that a particular array of six chromosomal markers was found in 97 of the 106 Cohens tested.
This collection of markers has come to be known as the Cohen Modal Hapoltype (CMH) -- the standard genetic signature of the Jewish priestly family.
Since the Y chromosome consists almost entirely of non-coding DNA (except for the genes determining maleness), it would tend to accumulate mutations.
Since it is passed from father to son without recombination, the genetic information on a Y chromosome of a man living today is basically the same as that of his ancient male ancestors, except for the rare mutations that occur along the hereditary line.
The chances of these findings happening at random is greater than one in 10,000.