COL1A2 and osteogenesis imperfecta: Another study has shown that some glycine substitutions caused by COL1A2 mutations can have such mild effects that they do not cause a detectable phenotype in the heterozygous state, but lead to OI only when homozygous.32 It is likely that more examples of partial OI phenotypes will be found when sequencing studies are more frequently performed in adults with a diagnosis of osteoporosis.