On average, a human cell contains about one billion molecules of protein which are members of an estimated 20,000 species. Some of these proteins live only minutes, others live months. Unlike damaged DNA, proteins cannot be repaired, but are degraded and replaced by new synthesis. To function properly, proteins need to be properly localized. To accomplish proper localization, membrane proteins need to be symmetrically integrated into cellular membranes and many soluble proteins require translocation across one or, in some cases, even two membranes. Translocation can be followed by container transport to distinct cellular compartments or to the outside of the cell.

Precise localization is accomplished by built-in sequence elements in each individual protein. Many proteins contain several such distinct sequence elements, termed topogenic sequences. Topogenic sequences are decoded by an array of cellular machineries. These include cognate signal recognition factors, membrane-specific docking proteins, protein-conducting channels and numerous other effectors, such as chaperones and many distinct Teases. Mislocalization of a protein often has severe consequences for its function and therefore is the cause of numerous diseases.