The R6/2 mouse is the most widely used animal model of Huntington's disease (HD), a genetic disorder causing movement disorders, personality changes, dementia, and premature death, for which there is currently no effective therapy. Use of animal models to assess novel therapeutic approaches to HD is Currently a major focus of research. Progress in this field will depend upon careful standardization of experimental protocols, and a sophisticated statistical approach. Here we investigate the sources of phenotypic variability in R6/2, and make recommendations for the future use of such models in therapeutic trials. (C) 2003 Elsevier Inc. All rights reserved.