The manifestation of the effects of a gene by the production of the particular protein, polypeptide, or type of RNA whose synthesis it controls. The transcription of individual genes can be ‘switched on’ or ‘switched off’ according to the needs and circumstances of the cell at a particular time. A number of mechanisms are responsible for the control of gene expression; the Jacob-Monod hypothesis describes a mechanism operating in prokaryotes (see operon). This involves the binding of specific regulatory proteins, called transcription factors, to control sequences in DNA. Other mechanisms operating in prokaryotes include regulation of translation to restrict protein synthesis, and modification or breakdown of transcription factors. Control of gene expression is more complicated in eukaryotes, which possess various control mechanisms not seen in prokaryotes. Eukaryotes have many more transcription factors than prokaryotes, with numerous interacting roles to play in different tissues and at different stages of development. Gene activity is controlled by multiple factors acting synergistically, and sometimes at considerable distance from the gene in question. Moreover, the packaging of DNA into chromatin also plays a part. Reversible chemical modifications of the histone proteins determine the degree of chromatin condensation and whether the genes are accessible for the transcription machinery (see chromatin remodelling). Also, many protein-encoding genes are regulated by small noncoding RNA molecules that bind to target mRNAs and degrade them or prevent their translation (see rna interference). 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; x inactivation.