The protein encoded by SAR1B (secretion associated Ras related GTPase 1B) is a small GTPase that acts as a homodimer. The encoded protein is activated by the guanine nucleotide exchange factor PREB and is involved in protein transport from the endoplasmic reticulum to the Golgi. This protein is part of the COPII coat complex. Defects in SAR1B are a cause of chylomicron retention disease (CMRD, also known as Anderson disease (ANDD). Two transcript variants encoding the same protein have been found for this gene.