ARCN1 encodes the coatomer subunit delta of coat protein complex I (COPI), which is essential for intracellular transport of type 1 collagen.1 Another example of deficient COPI transport of collagen is due to mutations in KDELR2 and shows a severe osteogenesis imperfecta (OI) phenotype.5 Furthermore, Izumi et al1 showed that mutations in ARCN1 not only impair intracellular collagen transport, but also trigger an endoplasmic reticulum (ER) stress response. Here, KDELR2 is linked to osteogenesis imperfecta.