The passage discusses familial DNA searching by police. It begins by imagining getting a knock on the door from police asking questions that lead to your brother being arrested for murder based on his DNA partially matching a sample from the crime scene. It then defines familial searching as using partial DNA matches to find potential close relatives of suspects. As an example, it discusses the arrest of the "Grim Sleeper" serial killer in Los Angeles through a partial DNA match to his offspring in a DNA database. While successful cases advocate for its use, only nine states currently allow familial searching due to privacy concerns.