Moonlighting proteins represent an interesting group of proteins which do not follow the classical formulation „one gene – one protein – one function“ by simultaneously performing two or more biochemically unrelated tasks. They do not include those proteins that have been genetically fused, alternatively spliced or proteolytically cut, or those that contain multiple functions within different domains or exert pleiotropic effect on phenotype. It's common for moonlighting proteins to be part of an essential metabolic pathway such as glycolysis, Krebs cycle, folding and stabilising of other proteins, transcription and translation, etc. There are plenty of mechanisms which moonlighting proteins use to switch between their canonical and alternative functions, of which the most important for this seminary is the execution of alternative function when outside of the cell, commonly attached to the plasma membrane. Biophysical fundaments on which moonlighting effect occurs include post-translational modifications, point mutations or gene duplications which make an alternate version of an original protein with an added moonlighting function. Bacteria use surface-associated moonlighting proteins to adhere to extracellular matrix of host epithelial cell, to activate plasminogen or even to evade host immune response by binding certain proteins of the immune system. Even to this day there are still no clues to how bacteria translocate their intracellular proteins outside of the cell without any signal sequence. The importance of research in the field of moonlighting is multifaceted – some have been observed to cause disease and cancer, others on the contrary make the web of metabolism more complex and intertwined by connecting distant pathways. The latter is very important to have in mind before attempting any systems biology approach.