Gene expression (also protein expression or often simply expression) refers to the multi-step process that begins with protein biosynthesis (spanning from the transcription of a gene into messenger RNA (mRNA) to its translation into protein) and is followed by folding, post-translational modification and targeting. The amount of protein that a cell expresses depends on the tissue, the developmental stage of the organism and the metabolic or physiologic state of the cell.
Indirectly, the expression of particular genes may be assessed with microarray technology, which can provide a rough measure of the cellular concentration of different mRNAs; often thousands at a time. While the name of this type of assessment is actually a misnomer, it is often referred to as expression profiling. (The expression of many genes is known to be regulated after transcription, so an increase in mRNA concentration need not always increase expression.)
The level of expression or activity of a gene depends in part on the expression levels of other genes called transcription factors, which often regulate that gene specifically.
Each gene thus may be regarded as a node in a network, with input - relevant proteins, and output the level of expression. The node itself is a function which can be obtained by combining basic functions, or gates, of the two outputs, which are now understood. It is similar to having boolean function computed using the basic AND OR and NOT gates in electronics. These functions carry information processing within cell and determine cellular behaviour. The basic drivers within cells are levels of some proteins, which determine both spatial (tissue related) and temporal (developmental stage) coordinate of the cell, and also its memory. The gene networks are only beginning to be understood, and it is a next fundamental step for biology - understanding nodal functions for each gene, and thus modeling behaviour of a cell.