Most important, they can't read the 3 billion letters--A, C, T and G, for the nucleotides adenine, cytosine, thymine and guanine--in the human genome from beginning to end, the way one reads a book.

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The four bases - adenine (A), guanine (G), thymine (T), and cytosine (C) - spell out the genes.

BBC: DNA pioneer is honoured

These symbols represent the four basic chemical letters, or bases, the body uses to form DNA--guanine, cytosine, adenine and thymine.

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