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transcription factor Any of a group of proteins that can regulate gene activity by increasing or decreasing the binding of RNA polymerases to the DNA molecule during the process of transcription. This is achieved by the ability of the transcription factors to bind to the DNA molecule (see DNA-binding proteins). Transcription factors contain finger domains, which are often in repeated sequences called multifinger loops. General transcription factors are active in the transcription of many genes. They bind to the promoter site near to the transcription start site and ensure the correct positioning of RNA polymerase with respect to the coding region of the gene. Regulatory transcription factors exert control over just one or a few genes and determine whether the gene is switched ‘on’ or ‘off’ by binding to regulatory sites, which may be located some distance from the coding region. They are crucial in ensuring that genes are expressed in a tissue-dependent manner and, during development, that genes are expressed at the appropriate time and place within the embryo. Hundreds of transcription factors may be involved in controlling just one gene.