An amazing aspect of the human genome is that there is so little variation in the DNA sequence when the genome of one person is compared to that of another. Of the 3.2 billion bases, roughly 99.9 percent are the same between any two people. It is the variation in the remaining tiny fraction of the genome, 0.1 percent--roughly several million bases--that makes a person unique. This small amount of variation determines attributes such as how a person looks, or the diseases he or she develops.