Mgp, or matrix gla protein, is closely related to osteocalcin (Bglap: “bone gla protein”), has been associated with chondrogenesis(112) and arthritis,(113) and promotes in vitro bone formation through Wnt signaling.(25) In agreement with our data, expression of Mgp has been found in both chondrocytes and vascular cells, with its function differing in each cell type.(114) Global loss of Mgp leads to premature death by 2 months of age through arterial calcification, so their true bone phenotype remains unknown. Here, MGP is linked to arthritic joint disease.