A biological signature left by a Goa'uld or Tok'ra symbiote that has died within the body of a surviving host. It is detectable by other blended beings, allowing a Goa'uld or Tok'ra to discern when a human was previously host to a symbiote.
In the Line of Duty - The Tok'ra Jolinar of Malkshur dies and is absorbed into the body of Samantha Carter, leaving behind a protein marker.
Legacy - Carter is unaffected by Ma'chello's Goa'uld killers, as they detect the protein marker left by a dead symbiote -- which she has. Fraiser, O'Neill and Teal'c are injected with her blood plasma to save them from the lethal technology.