Specific signal sequences are required for the translocation of proteins into and across both the endoplasmic reticulum of eukaryotes and the plasma membrane of prokaryotes. The similar properties of these signals, together with their ability to function when transferred between systems, suggested that the mechanisms of translocation in the two cases may be fundamentally similar. Indeed, recent findings have revealed striking similarities between essential components of the prokaryotic and eukaryotic translocation systems, suggesting that both are derived from a common ancestor.