Gene expression is the multi-step process in which genes create specific proteins or other substances in order to perform specific functions in organisms and their cells, including with regard to their structure, function and behavior. It is fundamental to life because genes exist in all living organisms, even including viruses.
All cells in an organism contain the same genes, but each cell typically expresses only a fraction of its genes, determined by a number of factors, both internal and external to the organism. They include the specific type of cell and its functions, communications from other cells, temperature, light, humidity, oxygen levels, chemicals, nutrition, exercise, stress, hormones and aging. When the gene expression malfunctions, such as due to damage from chemicals, radiation or other sources, cancer and other diseases can result.
As a simple example, one of the functions of the liver is to remove toxic substances such as alcohol from the bloodstream. To accomplish this, liver cells express genes which produce an enzyme that breaks down alcohol into non-toxic molecules. Likewise, in response to high blood sugar levels, the insulin gene is expressed to produce insulin, which functions to control blood sugar.