into advantages. The human genome is made up of three billion pairs of nucleotide molecules. Each of these contains one of four types of bases—abbreviated A, C, G and T—that represent a genomic alphabet encoding the information stored in DNA. Bases typically pair off according to strict rules to form the rungs in the ladderlike DNA structure. Because of these pairing rules, reading the sequence of bases along one half of the ladder reveals the complementary sequence on the other side as well. Our three-billion-base-long genome is broken into 23 separate chromosomes. People usually have two full sets of these, one from each parent, that differ by 0.01 percent, so that an