This special issue of Molecular and Cellular Biochemistry contains twenty-two selected research papers and reviews from a total of one hundred and ten presentations given at the 12th International Symposium on ADP-ribosylation Reactions: From Bacterial Pathogenesis to Cancer, held in Cancun, Mexico, May 10-14, 1997. The Symposium was hosted by the Sociedad Mexicana de Bioquimica and was sponsored by the University of North Texas Health Science Center, Fort Worth, TX, USA. This volume provides a state-of-the-art source of information for basic scientists and clinicians who are interested in the molecular, biochemical, and cellular aspects of protein-(ADP-ribose) transfer reactions in human health and disease.1. Poly ADP-Ribosylation: A DNA Break Signal Mechanism; F.R. Althaus, et al. 2. Measurement of Poly (ADP-ribose) Glycohydrolase Activity by High Resolution Polyacrylamide Gel Electrophoresis: Specific Inhibition by Histones and Nuclear Matrix Proteins; G. Pacheco-Rodriguez, R. Alvarez-Gonzalez. 3. Regulatory Mechanisms of Poly (ADP-Ribose) Polymerase; R. Alvarez-Gonzalez, et al. 4. Poly (ADP-Ribose) Polymerase: A Guardian of the Genome that Facilitates DNA Repair by Protecting Against DNA Recombination; S. Chatterjee, et al. 5. Trans-Dominant Inhibition of Poly (ADP-Ribosyl)ation Potentiates Alkylation-Induced Shuttle-Vector Mutagenesis in Chinese Hamster Cells; J. Tatsumi-Miyajima, et al. 6. Clostridial Toxins: Molecular Probes of Rho-Dependent Signaling and Apoptosis; D.A. Bobak. 7. Role of Brefeldin A-Dependent ATP-Ribosylation in the Control of Intracellular Membrane Transport; M.G. Silletta, et al. 8. A Dual Approach in the Study of Poly(ADP-Ribose) Polymerase: In Vitro Random Mutagenesis and Generation of Deficient Mice; C. Trucco, et al. 9. The RT6 (ART2) Family of ADP-Ribosyltransferases in Rat and Mouse; R. Bortell, et al. 10. Mapping the Role of NAD Metabolism in Prevention and Treatment ofl³