In molecular biology, transcription is the process of copying a segment of DNA into RNA, whereas a transcription factor (or TF) is a protein that controls the rate of copying this segment. The segments of DNA transcribed into RNA molecules that can encode proteins are said to produce messenger RNA (mRNA). The work of TFs is to turn genes on or off in order to ensure that they are expressed in the correct cells in the right amount, at the right time and in the right place.