SAR1B and chylomicron retention disease: In CMRD, most SAR1B mutations alter an amino acid in Sar1B's GDP/GTP binding pocket (2, 17, 18), suggesting that the intestinal fat malabsorption defect is attributable to breakdown/absence of the Sar1 GTPase cycle that normally secures capture of nascent chylomicrons into COPII pre-budding complexes and their subsequent onward transport.