Isochore assignment confidence and GC context. (a) Confidence as a function of the GC content of the genomic environment. Isochore assignment confidence is best in GC-poor regions; it decreases as the genomic context becomes more GC-rich and reaches a minimum around 55-58% GC. However, the assignment confidence becomes better again in the GC-richest regions with >59% GC. (b) Variance of the confidence depending on the GC content. The confidence variance is independent of the GC context for isochores with a GC content between about 33% and 62% GC, that is, for the main bulk of the genomic DNA.