P3H1 and osteogenesis imperfecta: Here, we move our attention from the extra- to the intracellular space and describe the effect of overmodified type I collagen on cellular homeostasis of seven recessive OI cases, three carrying mutations in CRTAP, three in P3H1 and one in PPIB, using skin fibroblasts in which no mutant protein was detectable (Fig. 1C).