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National Geographic is on the brink of solving a genetic mystery - was Abraham Lincoln dying of cancer when he was killed? Lincoln's Secret Killer: Revealed follows clues in photographs and relics, along with DNA testing, to shed light on this mystery. Meet with a diagnosis expert who is compiling a medical chart and see the first round of DNA testing- from a dress swatch said to be stained with Lincoln's blood. Meanwhile, in a DNA lab in New Zealand tries to crack the code.