XBP1 and osteogenesis imperfecta: Such a possibility is consistent with another report where a osteogenesis imperfecta COL1A2 G610C mutation causes procollagen I misfolding in the ER of osteoblasts and abnormal osteoblast differentiation, but did not induce the classical UPR mediators BiP or Xbp1-spliced form (Mirigian et al., 2016).