brigidmac
I've had a DNA match with a Jewish man in USA who is a 3rd to 6th cousin, but possibly 4th (although I think he's a generation younger than me) with a shared DNA match of 0.35%, and 26cM, the link Gadget gave states "Looking at the size of the biggest piece of DNA, you want that piece to be larger than 20cM" so I fit that category - just.
This is the piece of the jigsaw (or brick wall) I've been looking for to trace my g.grandfather's roots. I know none of my other roots are Jewish and I've always thought this part of my tree was Jewish so wasn't surprised to find this on the DNA.
The match in USA kindly gave me his paternal grandparents names (they were from London) and I've traced back to the early 1800s - there is a site in Holland which is very useful, also JewishGen.org which a Rootschatter told me about.
So far, on just one branch, I've come across quite a few cousins marrying each other, until the late 1800s. It is so much harder to trace Jewish ancestors, quite often they haven't registered the births and if marriages and burials are not on line of various Jewish sites, it would mean contacting the Bevis Marks synagogue in London - which I've not done yet. It will take me ages to research all the different branches from the early 1800s to the 1860s, all the marriages and children etc. and I'm just looking for the needle in a haystack who could have been the mother or father of my g.grandfather born about 1857/8. I doubt I'll ever find the parent, but at least I will have found out a lot more about the missing part of my tree.
Good luck with your search