Collagen alpha-2(I) chain is a protein, one of the chains for type I collagen, the fibrillar collagen found in most connective tissues. Mutations in this gene are associated with osteogenesis imperfecta, Ehlers-Danlos syndrome, idiopathic osteoporosis, and atypical Marfan syndrome. De Wet et al. (1987) isolated 60 kb of cloned DNA containing the entire COL1A2 gene and 22 kb of flanking sequences. Like the homologous avian gene, the 1,366 amino acid residues of the human prepropolypeptide chain are encoded by 52 exons. Analysis of the 5-prime and 3-prime untranslated regions conclusively established the nature of 5 polymorphic mRNA transcripts. The exons are equally distributed as follows: 6 in the N-propeptide domain, 42 in the alpha-chain region, and 4 in the C-propeptide domain.