Marfan syndrome (MFS) results from heterozygous mutations in FBN1. However, genetic analyses of deoxyribonucleic acid (DNA) from approximately 10-30% of MFS patients who meet diagnostic criteria do not reveal an identifiable FBN1 mutation. In a patient who met the diagnostic criteria for MFS, bidirectional DNA sequencing of exons and intron-exon boundaries of FBN1 failed to reveal a mutation. Assessment of the FBN1 message in dermal fibroblasts from the patient revealed insertion of a pseudoexon between exons 63 and 64. Sequencing of intron 63 identified a point mutation, IVS63+373, located near the middle of intron 63 of FBN1 that created a donor splice site in intron 63, leading to inclusion of a 93-bp fragment of intronic sequence in the FBN1 message. Identification of a novel pseudoexon mutation in FBN1, in association with a clinical diagnosis of MFS, confirms that cryptic mutations that are missed by the current DNA-based diagnostic methods have a causative role.