Associates with the organic matrix of bone and cartilage. Thought to act as an inhibitor of bone formation. In 16-day-old embryo, expressed at high level in the sternum and tibia, at low level in skeletal muscle, heart and gizzard. Not present in skin, liver and brain. Requires vitamin K-dependent gamma-carboxylation for its function. Belongs to the osteocalcin/matrix Gla protein family.