The last decade has witnessed rapid progress in our under- standing of the mechanisms of protein export and secretion in both prokaryotic and eukaryotic cells. Studies of protein secretion across the membranes of the rough endoplasmic reticulum have led to the formulation of the now-classic signal hypothesis, which has stimulated many discussions and new ideas, and the identification of the signal recogni- tion particle as an organelle in the initiation of the export process. However, more recent work pertaining to intrage- nic information related to targeting specific proteins for either...