Peter Rubinstein, senior managing director and head of commercial mortgage-backed securities research at Bear Stearns, has left the firm. He reported to Dale Westhoff, senior managing director of MBS research. Rubinstein's duties will be assumed by Alan Todd, associate director and CMBS researcher, according to Renu Aldrich, spokeswoman. Todd and Westhoff referred calls to Aldrich. Rubinstein could not be reached and his destination could not be determined by press time.