(covers information from several alternate timelines)
A quantum signature is a unique subatomic resonance which is constant across all matter in the universe. This signature cannot be changed by any known process, and can be considered to be the basic foundation of existence.
In 2370, Lieutenant Worf was thrown into a state of quantum flux as a result of an encounter with a quantum fissure, causing him to shift between different quantum realities. Detecting this flux in Worf's cellular RNA, which extended to the subatomic level, Lieutenant Commander Data was able to determine that Worf's quantum signature was not consistent with the resonance of matter in the rest of the universe, and that he had originated from a different quantum universe. (TNG: "Parallels")