A gene is usually taken to represent the genetic information transcribed into a single messenger RNA(mRNA) molecule, which in turn is translated into a protein. The locus of a gene is the region of the chromosome in which the gene is physically located. In diploid organisms, with paired homologous chromosomes and therefore pairs of genes, the two copies of the same gene are known as alleles. A human gene contains exonic regions, the sequence of bases that specify amino acids, interspaced by non-coding sequences called introns. Introns are of uncertain function, and are removed from the messenger RNA by a process called splicing. 

Within the transcriptional unit (the DNA region copied into mRNA), a regulatory promoter for transcription and a terminator site for translation are found. The promoter, a specific initiation point for transcription, encompasses a binding region fot RNA polymerase and is located upstream of the coding sequence. A stop site, terminator, for transcription is also required and found at the end of the transcriptional mit a downstream 3 region. Additional regulatory sequences are located outside or even within the gene, controlling where and when a particular gene is transcribed. These regulatory sequences are recognised by regulatory elements. 

Cis regulatory elements act only on adjacent genes, i.e. on the same chromosone, whereas trans- regulators are soluble molecules made by one gene which interact with genes on the same or different chromosomes. Promoters and enhancers are two types of cis-regulatory DNA sequences that influence which genes become transcribed in which cells and when. This process is termed the spatial and temporal control of gene expression. Enhancers are found on either side of the coding region and control the efficiency and rate of transcription from a particular promoter, by binding specific trans regulatory proteins called transcription factors. Transcription can either be stimulated or inhibited depending on the transcription factors present in the cell.

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