The human genome is a mosaic structure on many levels: there exist cytogenetic bands, GC composition bands (isochores) and clusters of broadly expressed genes. How might these inter-relate? It has been proposed that to optimize gene regulation, housekeeping genes should concentrate on transcriptionally competent chromosomal domains. Prior evidence suggests that regions of high GC and R bands are associated with such domains. Here we report that broadly expressed genes cluster in regions of high GC, and in R and lightest Giemsa bands. This is not only a confirmation of the adaptive hypothesis, but is also the first direct systematic evidence of a general interdependence of expression patterns with base composition and chromosome structure.