The frequently quoted stat that genetically all humans are 99.9 percent similar is of relevance here. That means that if we compare two individuals’ DNA, they differ by only one letter in a thousand. But if you factor in the three billion letters of code in a human’s genome, three million individual letters are different (and don’t include all manner of different, larger variations within our codes). That is a lot of variables to play with, and goes a long way to explain why we are all unique, even identical twins. Your precise genome sequence has never existed before and will never exist in another person.