After undergoing a bone marrow transplant for leukemia, Chris Long, a man from Nevada, experienced a remarkable genetic transformation. Following the procedure, he discovered through DNA tests that his blood — and even cells in his lips, cheeks, and semen — no longer carried his own DNA, but that of his donor. The phenomenon made him a genetic chimera: a person with two distinct sets of DNA. Over the course of four years, Long and his colleagues at the Washoe County Sheriff’s crime lab conducted