Transcription is regulated through the binding of transcription factor proteins to specific cis-level regulatory sites in the DNA. The nature of this regulation depends on the transcription factor. For example, some proteins activate transcription by recruiting RNA polymerase, some repress transcription by suppressing this recruitment, and others insulate proximal regions from the activity of nearby transcriptional activators or repressors. A key characteristic of each transcription factor protein is its DNA binding domain. Each DNA binding domain recognizes and interacts with DNA that matches a specific nucleotide pattern, or motif. These motifs tend to be short and degenerate, so even when the DNA binding motif is known, one cannot generally predict where a given transcription factor may bind. In general, transcription factor binding is determined experimentally.

These tracks contain transcription factor binding sites determined by ChIP-seq. This process involves fragmenting DNA, selecting the fragments of DNA that are bound by a certain transcription factor, and sequencing those DNA fragments. This generally yields a large library of DNA sequences, including some that were bound by the transcription factor directly, some that were bound indirectly via interactions with other molecules, and some false positives (such as cases of nonspecific binding). With the appropriate analysis methods, ChIP-seq can be a valuable approach for elucidating transcription factor binding and cis-level regulation.

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