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'''Transcription factors''' are proteins that bind to specific [[DNA]] sequences in order to [[gene regulation|regulate]] [[gene expression]].  They can act as activators or repressors, respectively increasing or decreasing the [[transcription]] of a target [[gene]].  Some transcription factors function by directly recruiting (or, conversely, blocking the recruitment of) [[RNA polymerase]] to the [[promoter]] region of a target gene; however, the majority of transcription factors function through interaction with one or more [[adapter protein|adapter proteins]].<ref>Weaver, Robert F. ''Molecular Biology''. 4th ed. Boston: McGraw-Hill, 2008.</ref>

'''Transcription factors''' are proteins that bind to specific [[DNA]] sequences in order to [[gene regulation|regulate]] [[gene expression]].  They can act as ''activators'' or ''repressors'', respectively increasing or decreasing the [[transcription]] of a target [[gene]].  Some transcription factors function by directly recruiting (or, conversely, blocking the recruitment of) [[RNA polymerase]] to the [[promoter]] region of a target gene; however, the majority of transcription factors function through interaction with one or more [[adapter protein|adapter proteins]].<ref>Weaver, Robert F. ''Molecular Biology''. 4th ed. Boston: McGraw-Hill, 2008.</ref>

Transcription factors comprise several [[gene family|protein families]] (groups of closely related proteins), many of which are found in every lineage of organism.  Genes encoding for transcription factors make up a significant portion of every organism's genome.  There are estimated to be well over 2000 genes (some estimates are as high as 3000) encoding for transcription factors in the human genome (~10% of all human genes)<ref>http://www.ncbi.nlm.nih.gov/pubmed/11823631</ref><ref>http://www.ncbi.nlm.nih.gov/pubmed/15193307</ref>.  These genes tend to be highly conserved and, in many instances, can be functionally interchangeable between organisms of different clades.

Transcription factors comprise several [[gene family|protein families]] (groups of closely related proteins), many of which are found in every lineage of organism.  Genes encoding for transcription factors make up a significant portion of every organism's genome.  There are estimated to be well over 2000 genes (some estimates are as high as 3000) encoding for transcription factors in the human genome (~10% of all human genes)<ref>http://www.ncbi.nlm.nih.gov/pubmed/11823631</ref><ref>http://www.ncbi.nlm.nih.gov/pubmed/15193307</ref>.  These genes tend to be highly conserved and, in many instances, can be functionally interchangeable between organisms of different clades.