Heidmann and his colleagues compared the variants of the virus-like sequence. It was as if they found four copies of a play by Shakespeare transcribed by four careless clerks. Each clerk might make his own set of mistakes, and sometimes one of Shakespeare’s words ended up as four different misspelled versions—wheregore, sherefore, whorefore, wherefrom. By comparing all four versions, a historian could figure out that the original word was wherefore.