In human genetics, mtDNA haplogroups and Y-DNA haplogroups are designated by letters of the alphabet. When a person takes a genealogical DNA test, the presence of an asterisk (*) in their test result indicates that they are a member of a particular haplogroup, but not of a known subclade (subdivision). Specifically, they do not possess any of the mutations that would place him/her in one of the known "downstream" subclades.
For example, a member of the Y-DNA Haplogroup R may belong to the sub-haplogroup R1 (defined by marker M173) or R2 (defined by marker M124). Individuals with neither of these mutations are categorised as belonging to group R*.