Gammacarboxyglutamic acid (Gla) is an abnormal amino acid, which occurs in a number of proteins. It was discovered about 10 years ago in the four vitamin K-dependent blood clotting factors and it could be demonstrated that Gla is formed in a post-translational modification step, which requires a carboxylating enzyme system (carboxylase) and vitamin K. Since at the time of this discovery the earlier mentioned clotting factors were the only proteins known to be synthesized in a vitamin K-dependent way, it has been assumed for many years that the blood clotting system was unique in this respect. Recently it has been demonstrated, however, that vitamin K-dependent carboxylase is not restricted to the liver (the place of synthesis of the clotting factors) but that it is also present in other tissues such as lung, kidney, spleen and testis. Moreover, numerous Gla-containing proteins have been detected, although in most cases their function is not wholly understood. It seems that (like for instance the glycosylation) the vitamin K-dependent carboxylation is a normal post-translational modification, which is required for the correct function of a certain class of Ca2+-binding proteins.