The human RAB genes share structural and biochemical properties with the Ras gene superfamily, and RAB4 (synonym: RAB4A) is a small GTPase belonging to this Ras superfamily. Accumulating data suggests an important role for RAB proteins either in endocytosis or in biosynthetic protein transport. The transport of newly synthesized proteins from endoplasmic reticulum to the Golgi complex and to secretory vesicles involves the movement of carrier vesicles, a process that appears to involve RAB protein function. RAB4 may also function as regulator along the receptor recycling pathway.