Each mtDNA location consists of a single base value of: A, C, T, or G. For instance, location 16073 might have a base value of "C".

Bases at different locations can randomly change from one value to another. For instance, a C might change to a T. This type of change is called a mutation.

mtDNA mutations are very rare. For this reason, we don't often see mtDNA mutations between maternal-line cousins in genealogical time (the last 500 years).

Most mtDNA tests cover a minimum of 400 locations, and SMGF tests over a thousand locations. Displaying base values at this many locations is impractical, so SMGF and other companies report base values based on the "CRS".

The CRS (Cambridge Reference Sequence) is a set of mtDNA locations and base values that is universally used as a reference. Test reports then show the differences with the CRS. Locations with no differences to the CRS are not listed.

For example, an mtDNA report might list your mutations as 16184T and 16399G. This means that your location 16184 had a different base value of "T" than the CRS, and your location 16399 had a different base value of "G" from the CRS.

Some mtDNA reports will abbreviate the locations. For instance, 16184T may simply be reported as 184T. This will be the case when the test company only reports results from region HVR1.

For companies that also test HVR2, the second set of results will always be in the 00000 series, but may also be abbreviated. For instance, 00073G may simply be reported as 073G.