In addition to SEC31A-related Halperin–Birk syndrome, four other Mendelian disorders have been linked to COPII subunits, including chylomicron retention disease due to allelic mutation in SAR1B, craniolenticulosutural dysplasia due to SEC23A mutations, congenital dyserythropoietic anemia type II secondary to SEC23B mutations, and autosomal recessive Cole–Carpenter syndrome 2, due to SEC24 complex defects [4,33,34,35]. Here, SEC23B is linked to chylomicron retention disease.