COL1A1 and osteogenesis imperfecta: Null mutations in the COL1A1 gene lead to quantitative defects in collagen production and are usually responsible for the mild form of osteogenesis imperfecta type I. This mild form can also be caused by point mutations in the COL1A1 gene, such as the substitution of glycine with cysteine at positions 43 and 46 (G>T), or the replacement of arginine with a stop codon (−237C>T) [10,11,12,13].