Members of the RAB protein family, such as RAB1B, are low molecular mass monomeric GTPases localized on the cytoplasmic surfaces of distinct membrane-bound organelles. RAB1B functions in the early secretory pathway and is essential for vesicle transport between the endoplasmic reticulum (ER) and Golgi (Chen et al., 1997 [PubMed 9030196]; Alvarez et al., 2003 [PubMed 12802079]).[supplied by OMIM, Jan 2009].