We present a large Turkish family with autosomal dominant inherited ulnar-mammary syndrome in which 10 affected family members, spanning three generations, were diagnosed. The phenotypic expression of the disease was found to be highly variable among the affected family members showing posterior-limb deficiencies and/or duplications, mammary-gland hypoplasia, apocrine dysfunction, dental and genital abnormalities. Mutation analysis of the TBX3 gene showed a novel one base-pair insertion at position 89 (designated 88_89insA) in the coding region. The mutation leads to a shift of the open reading frame and causes a premature truncation of the protein (M30fsX110). The truncated protein lacks almost all functional important parts of TBX3, most likely leading to a complete loss of functional protein. Our findings indicate that ulnar-mammary syndrome shows a wide range of phenotypes even within the same family and provide further evidence that haploinsufficiency of TBX3 is the disease-causing mechanism. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.