Variation patterns of introns' length, as a function of GC content. Each dot contains 1000 introns (A-D). The species in A-C were grouped based on ranges of intron lengths, e.g., up to 16000 bp for mammals. L1, L2, H1, H2 and H3 in D represent five GC-isochore families in human genome  which are different in GC content (0~0.37, 0.37~0.41, 0.41~0.46, 0.46~0.53, 0.53~1, respectively). Genes that cover two or more family regions were excluded in D. The x-axis scale in A – D is based on the actual GC content in different genomes.