A DNA sequence is the sequence of nucleotides in a DNA molecule. It is written as a succession of letters representing the primary structure of a DNA molecule or strand. If functional, such a sequence carries information for the sequence of amino acids in a protein molecule. The possible letters are A, C, G, and T, representing the four nucleotide bases of a DNA strand — adenine, cytosine, guanine, thymine. The sequences are printed next to one another, without gaps, as in the sequence AAAGTCTGAC.