SAR1B (GTP-binding protein SAR1b) is 198 amino acid protein. SAR1B is involved in transport from the endoplasmic reticulum to the Golgi apparatus. This protein is activated by the guanine nucleotide exchange factor PREB. It is involved in the selection of the protein cargo and the assembly of the COPII coat complex. Mutations in the SAR1B gene are associated with CMRD which is autosomal recessive disorder of severe fat malabsorption.