Distribution of transcription factor binding sites in gene promoter regions. The x-axis is the position index of gene promoter sequence, and the y-axis is the occurrence frequency of the binding sites across all promoters. Only genes with full length promoter sequences were included in calculation (approximately 14,000 sequences). A portion of the downstream sequence relating to transcriptional start site was also included. The position with index of 0 specifies the transcriptional start sites.