The operon model of gene regulation for repressible enzymes has one important difference from the inducible enzyme system. Therefore, if the corepressor is absent, transcription and subsequent translation occur. Whenever the end produce is present it combines with the repressor substance to form a molecule that can attach to the operator locus and terminate transcription. The functioning of the promoter locus is the same for both inducible and repressible enzyme synthesis. For example, steroid hormones enter the cell, pass into the nucleus, and produce their effects by turning on the transcription of specific genes, whereas the peptide hormones remain on the surface of the cells and produce their effects through the activation of preformed genes.
Human hemoglobin provides a very complicated instance of gene regulation.
Each hemoglobin molecule consists of two pairs of polypeptide chains. In early embryonic development the two types of chains are zeta and epsilon, but at about eight weeks after conception their production is terminated and two other types, called alpha and gamma, are produced. At birth, gamma production is replaced by production of so-called beta chains, for the rest of the individual's life, but alpha chains continue to be formed. Despite much research, neither the programming of the hemoglobin types nor their significance is yet known.