In molecular biology, a transcription factor is a protein that regulates the activation of transcription in the eukaryotic nucleus. Transcription factors localise to regions of promoter and enhancer sequence elements either through direct binding to DNA or through binding other DNA-bound proteins. They act by promoting the formation of the preinitiation complex (PIC) that recruits and activates RNA polymerase.

The regulation of transcription factors is a highly complex process as it is dependent upon a number of events, most notable of which are the presence of other DNA binding proteins (including other transcription factors) as well as local chromatin structure. Initial models, based on in vitro expreriments suggested that there is a definite assembly sequence of transcription factors dictated by the DNA sequence. It is, however, becoming incresingly obvious that the events leading to activation of transcription are dependent on a large number of factors and are higly intertwined. Furthermore epigenetic information present on DNA appears to play an important role in transcriptional activation.

(source: Wikipedia)