I have some additional information on P66. To net it out, I think P66 is of limited value in marking subclades in the Y DNA tree.
Here is an old post related to P66 by Thomas Krahn.
http://archiver.rootsweb.ancestry.com/th/read/GENEALOGY-DNA/2008-11/1226271823There is a Ysearch ID set up to represent this haplotype. It is DZRKP.
Another P66+ person turned up under R-L21*. His P66+ call has been verified, but he has close GD's (2 to 4 over 67) with people who are P66-, like just about everyone else. The surnames of the P66+ and P66- people are very similar and what surname origin sites consider Scottish.
This comes from FTDNA related to the new P66+ fellow.
P66 is likely not reliable because it is in a region that repeats. This means, for one thing, a positive result can easily revert to the ancestral state. For another thing, it means if someone were positive for it, it is possible it would not be "seen" correctly by any type of test other than direct sequencing. ..... Regardless of the ultimate P66 result, considering its potential instability, we would definitely advise *against* assuming a difference in result at this SNP precludes a relationship.
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