The genomes packed within such small containers are correspondingly limited, ranging from 2,000 nucleotides up to about 1.2 million. The genome of a mouse, by contrast, is about 3 billion nucleotides. It takes three nucleotide bases to specify an amino acid and on average about 250 amino acids to make a protein (though some proteins are much larger). Making proteins is what genes do; everything else in a cell or a virus results from secondary reactions. So a genome of just two thousand code letters, or even thirteen thousand (as for the influenzas) or thirty thousand (the SARS virus), is a
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