Fibrillin is a very large molecule whose primary structure is now known from the cloning and sequencing of 10 kb of cDNA. Immunohistochemical results suggest that one of the functions of fibrillin molecules is to contribute to the structure of the microfibril. The importance of fibrillin as a structural macromolecule has been demonstrated by the identification of the gene for fibrillin (FBN1) as the disease-causing gene in Marfan's syndrome. While it is clear that fibrillin contributes to the structure of the microfibril, it is not known whether fibrillin molecules self-assemble or whether fibrillin interacts with other molecules in order to form microfibrils. In order to investigate whether particular domains of fibrillin are important to the assembly of the microfibril and to specify domains that participate in interactions with other proteins, we have produced recombinant fibrillin 1 peptides in human cells and used them in studies described here. Additionally, new information regarding the 5' end of FBN1 has been obtained from studies investigating promoter activity, and potential proteolytic cleavage sites have been identified in the N- and C-terminal domains.