GC level distributions of a species' nuclear genome, or of its compositional 
fractions, encode key information on structural and functional properties of the 
genome and on its evolution. They can be calculated either from absorbance 
profiles of the DNA in CsCl density gradients at sedimentation equilibrium, or 
by scanning long contigs of largely sequenced genomes. In the present study, we 
address the quantitative characterization of the compositional heterogeneity of 
genomes, as measured by the GC distributions of fixed-length fragments. Special 
attention is given to mammalian genomes, since their compartmentalization into 
isochores implies two levels of heterogeneity, intra-isochore (local) and 
inter-isochore (global). This partitioning is a natural one, since large-scale 
compositional properties vary much more among isochores than within them. 
Intra-isochore GC distributions become roughly Gaussian for long fragments, and 
their standard deviations decrease only slowly with increasing fragment length, 
unlike random sequences. This effect can be explained by 'long-range' 
correlations, often overlooked, that are present along isochores.