The promoter region contains the DNA sequences that allow the formation of the transcription initiation complex that give the specificity of expression. Promoters are restricted to the 100-200 bp preceding the transcription start site. Enhancers have a global amplification effect; however, they do not increase promoter potency by enhancing the chance of having active transcription complex. Enhancers may reinforce tissue specificity of expression. In most cases, they are found in the 0.5-10 kb region preceding the promoter. Enhancers may be found in the transcribed region and after the transcription terminator, and they can be added to a construct in the region upstream of the promoter but also in the transcribed region, even within an intron. An increasing number of promoters are now becoming available. They may be chosen using published data to drive a cell specific expression in vivo.