It is very difficult to distinguish control elements in the DNA of higher eukaryotes, since they are mingled with vast amounts of non-functional sequence. In contrast, protein-coding exons are much easier to analyse. Exons are often extensive and compact. Exon regions can be searched for open reading frames and compared with peptide and cDNA sequences. Of control elements, only promoters, in fact only the TATA box and other elements that have defined locations relative to the transcription initiation site, can be characterized by their location.