Genome order index should not be used for defining compositional constraints in nucleotide sequences
A genome order index, defined as S=a2+c2+t2+g2, where a, c, t, and g are the nucleotide frequencies of A, C, T, and G, respectively, was used to suggest that there exist genome-specific constraints on nucleotide composition. We show that the evidence for constraint, S<1/3, is in fact a mathematical property that is always true regardless of data. Moreover, we show that S is strictly equivalent to and derivable from the Shannon H-function and has no advantage over it.
genome order index nucleotide composition shannon h-function genomic g+c content isochores