Co-expression of genes in clusters. (a,b) Expression of genes in the same cluster in different microarrays. (a) Cluster of four down-regulated genes (in green) in trx microarrays. (b) Cluster of four up-regulated genes (in red) in ash1 microarrays. Notice the boundaries and the co-regulated genes of the cluster are not the same in both experiments. These images were produced using the program GFF2PS . (c) Graphical comparison between co-expression of genes in trx and artificial clusters, according to the expression data provided in . For each cluster, the co-expression level was computed as the mean of Pearson's correlation coefficient between all pairs of genes in the cluster. The distribution of co-expression values within the boundaries of the trx clusters (including all genes or only the deregulated ones) is clearly skewed to the right, indicating much stronger co-expression than expected at random.