NPE comes from the genetic genealogy term “nonpaternity event,” another increasingly common term is “misattributed parentage.”
By constructing “mirror trees”—a technique genetic genealogists use to find common ancestors—one can narrow down possible NPE ancestors.
Identify close DNA cousins
Once your DNA results are available, you can view your DNA Matches. These are other people that have taken the DNA test that share DNA with you, and are likely genetic relatives of yours. Estimates of the relation of these DNA matches to you are categorized either as parent/child, close family, first cousin, second cousin, third cousin, fourth cousin, or distant cousin.
For men with Paternal NPE Ancestor(s), finding out specific yDNA designation can help narrow down things. For jewish men, this usually be group J or E.
Speaking of Jewish ancestry, due to endogamy, you want to only include matches that are closer than 2nd cousins.
You can be confident there is a common DNA ancestor when 3 people match over 10cm in the same spot on DNA.
At GEDmatch – the comparisons are easy. Just compare two kit numbers using the one-to-one utility to see if they match each other on the appropriate segment. The ones that do are Triangulated. You may also use the Tier1 Triangulation utility or the Segment utility.