The basal promoter An Analogy Hormones exert many of their effects by forming transcription factors.
Embryonic development requires the coordinated production and distribution of transcription factors.
The latest estimates are that a human cell, a eukaryotic cell, contains approximately 35,000 genes. Some of these are expressed in all cells all the time. Some are expressed as a cell enters a particular pathway of differentiation. Some are expressed all the time in only those cells that have differentiated in a particular way. For example, a plasma cell expresses continuously the gene for the antibody it synthesizes.