ER retention of type I collagen was observed in a mouse OI model, Aga2, with a dominant frameshift mutation in the collagen type I α1 C-propeptide domain (26) and a dachshund OI model with a mutation in the heat shock protein 47 (HSP47) gene, which encodes a chaperone for collagen biosynthesis in the ER (27). The gene discussed is SERPINH1; the disease is osteogenesis imperfecta.