MBTPS2 and osteogenesis imperfecta: Nevertheless, electron microscopy analyses of fibroblast cultures in vitro revealed the presence of abnormally shortened and enlarged ER cisterns in MBTPS2-OI and MBTPS2-IFAP/KFSD patient fibroblasts in contrast to longer and slender ER cisterns present in control fibroblasts (Figure 4), suggesting that pathogenic variants of MBTPS2 impose some form of cellular stress in both OI and IFAP/KFSD.