Rab 40B is a 278 amino acid protein that belongs to the small GTPase superfamily and the Rab family. Rab 40B contains a SOCS box domain that mediates interactions with the Elongin BC complex, an adapter module in different E3 ubiquitin ligase complexes. Rab 40B shares similarity with a yeast protein, suggesting a role in regulation of secretory vesicles. The Rab 40B gene is conserved in chimpanzee, cow, mouse, rat, chicken and zebrafish, and maps to human chromosome 17q25.3. It has been suggested that there is a major gene locus on chromosome 17q25.3 for autosomal dominant moyamoya disease (MMD), which is an idiopathic steno-occlusive cerebrovascular disease that represents an important cause of stroke.