Mutations in the gene for fibrillin-1 (FBN1) cause Marfan syndrome, a dominantly inherited disorder of connective tissue that primarily involves the cardiovascular, ocular, and skeletal systems. There is a remarkable degree of variability both within and between families with Marfan syndrome, and FBN1 mutations have also been found in a range of other related connective tissue disorders collectively termed type-1 fibrillinopathies. FBN1 mutations have been found in almost all of the 65 exons of the FBN1 gene and for the most part have been unique to one affected patient or family. Aside from the "hot spots" for the neonatal Marfan syndrome in exons 24-27 and 31-32, genotype-phenotype correlations have been slow to emerge. Here we present the results of temperature-gradient gel electrophoresis analysis of FBN1 exons 59-65. Six mutations were identified, only one of which had been previously reported. Two of the six mutations were found in patients with mild phe-notypes. Taken together with other published reports, our results suggest that a sizable subset (ca. 40%) of mutations in this region is associated with mild phenotypes characterized by the lack of significant aortic pathology, compared with about 7% in the rest of the gene. In two cases, mutations affecting analogous positions within one of the 43 cbEGF modules of FBN1 are associated with mild phenotypes when found in one of the 6 C-terminal modules (encoded by exons 59-63), but are associated with classic or severe phenotypes when found in cbEGF modules elsewhere in the gene. Am.