COL1A1 and osteogenesis imperfecta: Although more than 85% of OI cases can still be traced to pathogenic variants in either of the two collagen type I–coding genes COL1A1 or COL1A2 (112, 113), the several other genes identified over the past 12 years in OI or monogenic forms of primary osteoporosis need to be kept in mind (92, 114, 115).