Analysis in yeast of the relationship between a gene's genomic position and its expression profile, derived from chip array data, suggests that both closely linked genes and genes spaced at regular intervals show correlated expression profiles. Unfortunately, yeast arrays are often printed in genomic order. The above results may hence reflect little more than known spatial biases within arrays. To circumvent this problem, we analyse spatially unbiased expression data derived from a large Northern blot study. We find that local domains of co-expressed genes range up to 30 genes (100 kb), and are thus much larger than previously considered. There is, by contrast, no evidence for periodicity of co-expression in yeast. We likewise find no convincing evidence for periodicity in the human or mouse genome. Further, analysis of yeast transcription factor binding data sets suggests that there is currently no statistical evidence for chromosomal periodicity of co-regulation. (c) 2006 Elsevier Ltd. All rights reserved.