mtDNA haplogroup U3 is present throughout Europe and Western Asia but is found at the highest frequencies in countries surrounding the Black Sea, especially in Bulgaria and the Caucusas. U3 is very rare in Western Europe (probably less than 0.5%) but is found at increasingly high levels to the east, especially in the Danube River basin and along the southeastern Mediterranean Sea. U3 is also found at very high frequencies among some Roma populations and in some parts of the Near East (e.g. Jordan and Syria). U3 appears to be a fairly ancient haplogroup (probably at least 35,000 to 45,000 years old). mtDNA haplogroup U3 is a subclade of haplogroup U. All members of haplogroup U3 are expected to have the basic characteristics of U plus four differences that define U3. Customers of FTDNA can expect that they will have been tested for a coding region SNP (likely 12308G) before being assigned to haplogroup U or any of its subclades. Additionally, any customer in haplogroup U who has HVR2 results should observe that they have at least the following: 073G, 263G, and 315.1C. From there, you will be assigned to a subclade of haplogroup U based on your HVR1 test results. Every member of U3 should expect to observe 16343G - sometimes abbreviated to simply "343G", since this is one of the four defining markers of haplogroup U3. A second one is 150T, which is in HVR2. The remaining two (14139G and 15454C) are in the coding region and would only be revealed with the purchase of a full genenomic sequence. Haplogroup U3 can be further bifurcated into two subclades, U3a and U3b. The distinction is largely made in the coding region, but U3a is almost always detectable by the presence in the HVR1 region of 16390A (or simply "390A". U3a appears to be the most populous subclade, though most of the Roma populations mentioned earlier contain only a lineage of U3b.