Relationship between motif density and functionality. Each point corresponds to a PAC/TOD6 motif occurrence (HGMGATGAG) in a promoter region of a S. cerevisiae gene. The x-axis represents the distance in bp of an occurrence from the start codon. The y-axis represents the motif density around the gene where the occurrence is found. Density is estimated by considering the number of genes containing the PAC motif in the neighborhood of each gene (Gasch et al. dataset) , and is expressed as a z-score (see motif density in Material and methods).