We are pursuing several lines of research: the characterization of mutations in type I collagen genes (COL1A1 and COL1A2) that give rise to forms of osteogenesis imperfecta and other disorders, the identification and characterization of mutations in the type III collagen gene (COL3A1) which give rise to Ehlers-Danlos syndrome type IV, characterization of mutations in other genes (e.g., COL5A1, COL5A2, PLOD1 and the N-terminal procollagen protease) that result in other forms of connective tissue disorder, identification of proteins in the intracellular and extracellular processing pathways that identify abnormal collagen proteins, and the mechanisms of mRNA processing in collagen genes to predict the outcome of splice site mutations. In addition, we are searching for other genes that may give rise to phenotypes of osteogenesis imperfecta, and determining the rate and genetic basis of parental mosaicism for mutations in these genes. The majority of mutations in the COL1Al and COLlA2 genes that cause OI result in substitution for glycines within the triple helix. Most of the remainder alter splice sites. Our studies of the mutations suggest that in some instances the order of exon splicing may determine the effects of splice mutations; as a consequence we are studying the order of intron removal in such cell strains. One of the most puzzling aspects of OI has been the failure to identify mutations in all affected individuals. Using long amplification regions, we have noted low level splice defects in some such patients that result in the production of only a small amount of abnormal molecules due to the presence of 5-10% abnormal mRNA species as a consequence of mutations outside the canonical splice site sequences. However, it is clear that some mutations reside outside these two gene.