Abstract Here, we present the first two Swedish cases of Conserved Oligomeric Golgi complex subunit 6‐congenital disorders of glycosylation (COG6‐CDG). Their clinical symptoms include intellectual disability, Attention Deficit/Hyperactivity Disorder (ADHD), delayed brain myelinization, progressive microcephaly, joint laxity, hyperkeratosis, frequent infections, and enamel hypoplasia. In one family, compound heterozygous variants in COG6 were identified, where one (c.785A>G; p.Tyr262Cys) has previously been described in patients of Moroccan descent, whereas the other (c.238G>A; p.Glu80Lys) is undescribed. On the other hand, a previously undescribed homozygous duplication (c.1793_1795dup) was deemed the cause of the disease. To confirm the pathogenicity of the variants, we treated patient and control fibroblasts with the ER‐Golgi transport inhibitor Brefeldin‐A and show that patient cells manifest a significantly slower anterograde and retrograde ER‐Golgi transport.
