The mammalian genome is not a random sequence but shows a specific, 
evolutionarily conserved structure that becomes manifest in its isochore 
pattern. Isochores, i.e. stretches of DNA with a distinct sequence composition 
and thus a specific GC content, cause the chromosomal banding pattern. This 
fundamental level of genome organization is related to several functional 
features like the replication timing of a DNA sequence. GC richness of genomic 
regions generally corresponds to an early replication time during S phase. 
Recently, we demonstrated this interdependency on a molecular level for an 
abrupt transition from a GC-poor isochore to a GC-rich one in the NF1 gene 
region; this isochore boundary also separates late from early replicating 
chromatin. Now, we analyzed another genomic region containing four isochores 
separated by three sharp isochore transitions. Again, the GC-rich isochores were 
found to be replicating early, the GC-poor isochores late in S phase; one of the 
replication time zones was discovered to consist of one single replicon. At the 
boundaries between isochores, that all show no special sequence elements, the 
replication machinery stopped for several hours. Thus, our results emphasize the 
importance of isochores as functional genomic units, and of isochore transitions 
as genomic landmarks with a key function for chromosome organization and basic 
biological properties.