COL1A1 and osteogenesis imperfecta: However, the lack of a classical UPR activation (e.g., elevated BiP levels) was previously reported in skin fibroblasts of Osteogenesis imperfecta patients with mutations in triple helical domain of COL1A1 [60], as well as in embryonic fibroblasts from a Marfan syndrome mouse model [61] and in patients’ fibroblasts with different types of mucopolysaccharidoses [62].