FBN1 and Marfan syndrome: Marfan syndrome (MFS [MIM 154700]) is remarkable for its phenotypic variability and its evolving definition.1,2 Limiting the definition of MFS patients to individuals with heterozygous pathogenic variants in the FBN1 gene has not reduced phenotypic variability as much as anticipated.1 Pathogenic variants in the FBN1 gene have been associated with phenotypes ranging from normal (incomplete penetrance is rare but possible) to severe MFS in childhood with cardiovascular, ophthalmologic, skeletal, cutaneous, and neurologic features, and shortened life expectancy.