Intracellular signalling peptides and proteins are peptides and proteins involved in transmission of signals (signal transduction) across the membrane of cells. Signals in the form of second messengers, hormones, proteins and other biological molecules are recognized by cell surface receptors, which mediate their responses to other intracellular signalling peptides and proteins, such as transcription factors. Many other intracellular proteins have also been found on the surface of bacterial pathogens and other organisms in proteomics experiments. It is quite possible that many of those proteins also have a moonlighting function on the cell surface. The increasing number and variety of known moonlighting proteins suggest that there may be more moonlighting proteins than previously thought, and moonlighting might be a common feature of many more proteins.The proteins in this study were selected with the criterion that the proteins have a biochemical function inside the cell and a second biochemical function on the cell surface. The proteins were identified by searching the literature for experimental evidence that each protein is performing a function on the surface, and is not just observed to be present on the surface.