On FTDNA I match two other Stephens males with the exception of 1 marker over 67 markers. When the three of us uploaded our DNA to Gedmatch, mine from FTDNA Family Finder and theirs from Ancestry, we got:
Minimum threshold size to be included in total = 700 SNPs Mismatch-bunching Limit = 350 SNPs Minimum segment cM to be included in total = 7.0 cM
Largest segment = 0.0 cM Total of segments > 7 cM = 0.0 cM No shared X-DNA segments found
Does this mean we are not related?
The lack of shared X-DNA segments is to be expected and does not imply you are unrelated. Men get their X chromosome from their mother and their Y chromosome from their father. If two men had the same father but unrelated mothers, they too would have closely matching Y chromosomes but no X-DNA match.
There's more to the story, such as that men's single X chromosome is a mixture of the DNA in their mother's two X chromosomes, whereas their father's single Y chromosome is passed down unchanged (except for occasional mutations, which are why all the men in the world don't have the same Y DNA). If you'd like to learn more, see Roberta Estes' blog post X Marks the Spot.
As previously indicated, the lack of matching X-DNA is not significant. A perfect y-DNA match implies a MRCA within 5-6 generations, a single mismatch implies a MRCA within 10-12 generations (roughly speaking). Once you get a match this close, you might want to upgrade to a higher-marker-count test to check further.
However, the lack of other matching DNA suggests that this match is quite distant, at least 4th cousins and probably further than that. Autosomal DNA really only works out to about 3rd (maybe 4th) cousins. There's only about a 50% chance of 2 randomly-chosen 4th cousins sharing any autosomal DNA at all, and the likelihood drops rapidly after that.
Your 67-marker perfect match could still be, say, a 5th or 6th cousin, and the 1 marker mismatch is very likely more distant than that.
