COL1A2 and osteogenesis imperfecta: The vast majority of OI cases are caused by pathogenic variants in COL1A1 and COL1A2 genes, whereas the remaining ~ 10–15% of cases are associated with pathogenic variants in one of the more than 20 OI-causative genes; OI types 1 to 4 are genetically very heterogeneous being caused by an increasing number of pathogenic variants in numerous OI genes whereas OI type 5 is caused by one variant in the IFITM5 gene [2, 17].