A Nonparametric Bayesian Modeling Approach for Cytogenetic Dosimetry Athanasios Kottas Institute of Statistics and Decision Sciences, Duke University Durham, NC 27708-0251, USA email : thanos@stat.duke.edu M´ arcia D. Branco Departamento de Estat ´ istica, IME, Universidade de S˜ ao Paulo Cxa Postal 66281, 05315-970, S˜ ao Paulo, SP, Brasil email : mbranco@ime.usp.br and Alan E. Gelfand Department of Statistics, University of Connecticut Storrs, CT 06269-4120, USA email : alan@stat.uconn.edu KEY WORDS: Auxiliary variables; Dirichlet process; Dose-response; Logistic regression; Polytomous response. 1