Studies on the vitamin K-dependent carboxylase are still in their infancy, but the cloning and purification of the enzyme have permitted a number of advances in our understanding of both the molecular biology, and mechanism of the carboxylase. Advances in our knowledge of the molecular biology of the carboxylase include chromosomal location, characterization of messenger RNA transcript(s) and the study of patients with carboxylase mutations. Our understanding of the mechanism of the carboxylase has been enhanced by the expression of peptides in E. coli which contain multiple carboxylation sites and more closely resemble the native carboxylase substrates. These peptides have been utilized to identify elements within the substrates which are critical for carboxylation and to demonstrate that the vitamin K-dependent carboxylase is one of the first examples of a processive post-translational modification enzyme.