The dietary essentiality of vitamin K was discovered as the result of a series of experiments carried out by the Danish nutritional biochemist Henrik Dam, working at the University of Copenhagen. Dam’s group succeeded in preparing a crude plasma prothrombin fraction from chick plasma and demonstrated that its procoagulant activity was decreased when it was obtained from vitamin K-deficient chicks. Dam collaborated with Karrer of the University of Zurich in the isolation of the vitamin, and by 1939 they had succeeded in isolating the vitamin as yellow oil from alfalfa. Research activity directed toward an understanding of the role of the vitamin K-dependent proteins that had been identified and the function of vitamin K in their synthesis increased in the early 1960s as an understanding of mammalian protein synthesis became available. The elucidation of the structure of vitamin K was an extremely competitive research area with a number of large groups involved.