The autosomal-dominant giant platelet syndromes (Fechtner, Epstein, and Sebastian platelet syndromes and May-Hegglin anomaly) represent a group of disorders characterized by variable degrees of macrothrombocytopenia with further combinations of neutrophil inclusion bodies and Alport-like syndrome manifestations, namely, deafness, renal disease, and eye abnormalities. The disease-causing gene of these giant platelet syndromes was previously mapped by us to chromosome 22. Following their successful mapping, these syndromes were shown to represent a broad phenotypic spectrum of disorders caused by different mutations in the nonmuscle myosin heavy chain 9 gene (MYH9). In this study, we examined the potential role of another gene, fibulin-1, encoding an extracellular matrix protein as a disease modifier. Eight unrelated families with autosomal-dominant giant platelet syndromes were studied for DNA sequence mutations and expression of the four fibulin-1 splice variants (A–D). A mutation in the splice acceptor site of fibulin-1 exon 19 was found in affected individuals of the Israeli Fechtner family, whereas no MYH9 mutations were identified. Unexpectedly, fibulin-1 variant D expression was absent in affected individuals from all eight families and coupled with expression of a putative antisense RNA. Transfection of the putative antisense RNA into H1299 cells abolished variant D expression. Based on the observation that only affected individuals lack variant D expression and demonstrate antisense RNA overexpression, we suggest that these autosomal-dominant giant platelet syndromes are associated, and may be modified, by aberrant antisense gene regulation of the fibulin-1 gene. Am. J. Hematol. 74:254–262, 2003. © 2003 Wiley-Liss, Inc.