Most OI cases are caused by qualitative or quantitative defects in COL1 synthesis: the vast majority of autosomal-dominant pathogenic variants (roughly 85–90%) affect the COL1A1 and COL1A2 genes, yet all mechanisms involved in collagen type I biosynthesis are known to be linked to OI pathophysiology, from its synthesis, modification and folding to its secretion into the ECM. This evidence concerns the gene COL1A1 and osteogenesis imperfecta.