Volume 4 of Advances in Nutritional Research reflects the increased importance that recently has been attached to nutrition in many fields of clinical medicine. This heightened interest in nutrition stems from the demonstration that the intake of specific nutrients may have far-reaching consequences, not only for normal metabolism, but also for metabolic processes affecting clinical or subclinical disease. Conversely, many disease states have been shown to have previously unrecognized effects on nutrient function and metabolism. In addition to topics of obvious relevance to human clinical nutrition, this volume contains chapters dealing with the nutrition of cells grown in culture and of species that may provide insights into nutritional disorders of man. To gether with its predecessors, Volume 4 provides graduate students and estab lished investigators with authoritative accounts of the status of research on a range of topics of current interest in experimental and clinical nutrition. vii Contents Chapter 1. Vitamin-Responsive Genetic Abnormalities ………. . S. Harvey Mudd 1. Introduction………………………………………. 1 2. Historical Perspective ……………………………….. 2 3. Genetic Abnormalities Currently Known to Be Vitamin Responsive. 3 4. Mechanisms Underlying Vitamin Responsiveness… … … … … . 7 4.1. Defects in the Processing of a Vitamin Prior to Its Interaction in Cofactor Form with a Particular Apoenzyme …….. 7 4.2. Defects in Particular Apoenzymes that Interact with the Cofactor Forms of Vitamins… … … … … … 16 … …