Dr Klein is a US board-certified gastroenterologist. Before joining the pharmaceutical industry he was on the GI faculties of the medical schools at the Oregon Health Sciences University, Stanford University, and the University of North Carolina. His primary research focus was brain-gut interactions.

He has had a longstanding interest in the functional GI disorders and has written and spoken widely on clinical trial design in the irritable bowel syndrome. His article, Controlled treatment trials in the irritable bowel syndrome: A critique (Gastroenterology, 1988-see publications page) is considered the seminal work on the design and evaluation of treatments for IBS. He was a member of the "Rome II" subcommittee on the design of treatment trials for the functional GI disorders.

His invited lectures on various aspects of the functional GI disorders include those at the World Congress of Gastroenterology (Vienna), The Royal College of Physicians (London), the National Institutes of Health (Bethesda), the International Symposium of Functional Gastrointestinal Disorders (Milwaukee, WI), the International GI Motility Society (Gmunden, Austria), the International Symposium on the Approach to the Patient with Chronic Gastrointestinal Disorders (Baveno-Stressa, Italy), and the Consensus Conference on Establishing Guidelines for the treatment of IBS for Canadian Family Physicians (Kingston, Ontario).

He has consulted to regulatory agencies, the National Institutes of Health, and the UK National Health Service as well as to numerous pharmaceutical companies on issues related to treatment trial design and evaluation in the functional GI disorders