This volume represents the Proceedings of the Twenty-Sixth Annual Biology Division Research Conference held April 9-12, 1973 in Gatlinburg, Tennessee. The subject of the symposium was Molecular Cytogenetics and the aim of the meeting was to bring together researchers interested in problems of chromosome organi- zation, activity and regulation in prokaryotes and eukaryotes. Cytological, biochemical and genetic approaches to these questions were included since the collective information gained from these disciplines provides an integrated approach to genome structure and function. The meeting was sponsored by the Biology Division of the Oak Ridge National Laboratory*. It would not have been possible with- out the interest and cooperation of the organizing committee under the chairmanship of O. L. Miller, Jr. Special thanks are due to the chairmen and speakers for making this volume possible and to Dr. Waldo Cohn for his assistance in editing. Preparation of the completed volume was due in large part to the efforts of Sandra Vaughan of the Biology Division. The Editors Barbara Ann Hamkalo John Papaconstantinou August, 1973 * Operated by the Union Carbide Corporation for the U.S. Atomic Energy Commission.