Osteocalcin (OC), also known as bone gla protein (BGP), is a 49 amino acid vitamin K-dependent calcium binding protein with a molecular weight of 5.9 kDa. Carboxylated OC (Gla-OC), which is capable of binding calcium, is the active form of the protein and decarboxylated OC represents the inactive form. Gla-OC has three γ-carboxylated glutamic acid residues (Gla) at positions 17, 21, and 24 that are known to mediate strong binding to hydroxyapatite. OC synthesis is dependent on both vitamins D and K. Vitamin D directly induces OC synthesis and vitamin K stimulates the gamma-carboxylation of glutamic acid residues. During osteoclastic resorption, binding of undercarboxylated osteocalcin (Glu-OC) to bone decreases, resulting in an increase in Glu-OC level in blood and urine. P. D. Delmas and colleagues have shown that undercarboxylated OC is significantly increased in elderly women, suggesting an age-dependent impairment of OC γ-carboxylation. The presence of undercarboxylated OC in serum has also been associated with changes to the bone matrix and increased fragility.