Genes that are expressed together are often found in clusters within the genome. In an Advanced Online Publication in Nature Genetics Csaba Pal and Laurence Hurst ask why genes might exist in clusters (Nature Genetics, 10 February 2003; DOI:10.1038/ng1111). They analysed the yeast Sacchraomyces cerevisiae looking specifically at genes that are essential for life. They found that indispensable genes are highly clustered within the genome. The clusters may be up to 20 genes long and differ from clusters of co-expressed genes. Clusters of essential genes have a much lower rate of recombination than the rest of the genome. The authors conclude that gene order and recombination rate may have co-evolved.