Three remarks on terminology

I will not use in this book the expression GC content, preferring GC level or, simply, GC (which is defined as the molar ratio of guanosine and cytidine in DNA see Abbreviations). Indeed, one can talk about a content only if there is a container. In the case of DNA, the nucleotides are not contained in DNA, they form DNA. 

The counterpart of AT is not CG (as used by some authors) but GC, because what matters here is not the alphabetical order but the purine-pyrimidine order. 

Finally, let me stress that by structural genomics I mean what used to be called nucleotide sequence organization or genome organization and not, as recently suggested (see, for example, Stevens et al., 2001 Baker and Sali, 2001), protein structure (although the latter also enters into the picture). 

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