Each intracellular compartment involved in the biosynthetic/secretory pathway of eukaryotic cells bears at its surface at least one small GTP-binding protein. Most of them belong to a distinct branch of the p21ras superfamily, the Sec4/Ypt1/rab family. Other proteins are members of the ARF family. They play a key role in the regulation of budding and targeting/fusion events occurring during protein transport.