Bernard-Soulier Syndrome (BSS) is caused by mutations inthe genes for platelet glycoprotein complex GPIb/V/IX. A 4-year-old boy with developmental delay experienced lifethreateningmucosal bleeding episodes triggered by trivialrespiratory infections. Giant platelets were present andristocetin-induced platelet aggregation was reduced, suggestingsevere BSS. Immune transmission electron microscopyshowed reduced expression of GPIb on thrombocytes. PCRand Southern analysis demonstrated that exon 1 and part ofexon 2 of GPIbβ was deleted on both alleles. Surprisingly,mapping of the deletion breakpoints revealed that the deletionextended 5′ from exon 1 of GPIbβ to include the whole genecoding for SEPT5 (septin 5). After allogeneic bone marrowtransplantation bleeding symptoms subsided while developmentaldelay persisted.In non-dividing cells (i.e. thrombocytes and neurons) septinsare implicated in exocytosis, vesicle trafficking and activemembrane movement. The unusual combination of BSS withmental retardation might be related to the functional deactivationof the two adjacent genes GPIbβ and SEPT5 and thusconstitute a novel contiguous gene syndrome. The contributionof SEPT5 deficiency to the severe bleeding symptomsand to developmental delay remains, however, to be proven.