The Ras-related superfamily of guanine nucleotide binding proteins includes the R-Ras, Rap, Ral/Rec and Rho/Rab subfamilies all of which are thought to play an important role in either endocytosis or in biosynthetic protein transport. The process of transporting newly synthesized proteins from the endoplasmic reticulum (ER) to various stacks of the Golgi complex and to secretory vesicles involves the movement of carrier vesicles and requires Rab protein function. Rab proteins are also an integral part of endocytic pathways. Rab 31, also known as RAB22B, is a 194 amino lipid-anchored protein that localizes to the cytoplasmic side of the cell membrane and belongs to the Ras-related GTPase superfamily. Expressed at high levels in lung, brain and heart, Rab 31 may function in a similar manner to other Rab proteins, namely playing a role in protein transport.