Keutel syndrome (KS, MIM 245150) is an autosomal recessive disorder characterized by abnormal cartilage calcification, peripheral pulmonary stenosis and midfacial hypoplasia. A genome search using homozygosity mapping provided evidence of linkage to chromosome 12p12.3-13.1 (maximum multipoint lod score, 4.06). MGP was a candidate on the basis of its localization to this chromosomal region and the known function of its protein. MGP maps to chromosome 12p near D12S363. Human MGP is a 10-kD skeletal extracellular matrix (ECM) protein that consists of an 84-aa mature protein and a 19-aa transmembrane signal peptide. It is a member of the Gla protein family, which includes osteocalcin, another skeletal ECM protein, and a number of coagulation factors (factors II, VII, IX, X and proteins S and C). All members of this family have glutamic acid residues modified to gamma-carboxyglutamic acids (Gla) by a specific gamma-carboxylase using vitamin K as a cofactor. The modified glutamic acid residues of Gla proteins confer a high affinity for mineral ions such as calcium, phosphate and hydroxyapatite crystals, the mineral components of the skeletal ECM. The pattern and tissue distribution of Mgp expression in mice suggest a role for Mgp in regulating ECM calcification. Mglap-deficient mice (Mglap-/-) have been reported to have inappropriate calcification of cartilage. Mutational analysis of MGP in three unrelated probands identified three different mutations: c.69delG, IVS1-2A-->G and c.113T-->A. All three mutations predict a non-functional MGP. Our data indicate that mutations in MGP are responsible for KS and confirm its role in the regulation of extracellular matrix calcification.