XBP1 and osteogenesis imperfecta: It remains unknown whether Xbp1 splicing, Atf6 proteolysis, or Eif2α phosphorylation occur in other mouse models of skeletal dysplasias characterized by ER stress, including the p.V194D Matn3 mutant model of multiple epiphyseal dysplasia [39], and an Aga2-mutant model of osteogenesis imperfecta [40].