Marfan syndrome is an autosomal dominant connective tissue disorder caused by mutations in the fibrillin-1 gene (FBN1). Penetrance of FBN1 mutations is complete but intra and inter familial clinical expressivity is extremely variable. The underlying mechanisms for variability are not understood. An interesting mechanism is that the expression level of the wild type and/or mutated allele may play a role in the determination of variability.