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gene expression The manifestation of the effects of a gene by the production of the particular protein, polypeptide, or type of RNA whose synthesis it controls. Individual genes can be ‘switched on’ (exert their effects) or ‘switched off’ according to the needs and circumstances of the cell at a particular time. A number of mechanisms are thought to be responsible for the control of gene expression; the Jacob–Monod hypothesis postulates the mechanism operating in prokaryotes (see operon). Control of gene expression is known to be more complicated in eukaryotes, which possess various control mechanisms not seen in prokaryotes. For example, the methylation of cytosine bases of specific genes in eukaryotic DNA is observed in cells in which the gene is not expressed; if DNA methylation is prevented by the use of inhibitory chemicals, this can cause certain genes to be expressed. In multicellular organisms, expression of the right genes in the right order at the right times is particularly crucial during embryonic development and cell differentiation. This involves subtle and complex interplay of chemical signals with the embryo's genes, in patterns that vary between different types of organism (see differentiation). Abnormalities of gene expression may result in the death of cells, or their uncontrolled growth, as in cancer. See also transcriptomics.