| Although the promoter is essential for initiation of transcription, it is not necessarily alone in influencing the strength of transcription of a particular gene. Another group of elements, known as enhancers, can regulate the level of transcription of a gene but, unlike promoters, their position may vary widely with respect to the startpoint and their orientation has no effect on their efficiency. Enhancers may lie upstream or downstream of a promoter and may be important in conferring tissue-specific transcription. For instance, a nonspecific promoter may initiate transcription only in the presence of a tissue-specific enhancer. Alternatively, a tissue-specific promoter may initiate transcription, but with a greatly increased efficiency in the presence of a nearby enhancer that is not tissue-specific. In some genes, for example immunoglobulin genes, enhancers may actually be present downstream of the startpoint of transcription, within an intron of the gene being actively transcribed.
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