The likelihood that a particular DNA segment will be attacked by the endonuclease enzyme DNase I, which breaks down DNA into smaller fragments by cleaving its phosphodiester bonds. DNA segments which contain active genes that are regularly transcribed are more prone to being cut apart by DNase I than inactivated genes, because the inactivated genes are generally within condensed regions of the chromosome, while the active genes are in more open configurations for easy access for transcription.

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