A key expert for Monsanto in numerous trials over its Roundup pesticide was in federal court Tuesday defending his research, which attempts to answer the question: What causes cancer?

Dr. Cristian Tomasetti, director of the Center for Cancer Prevention and Early Detection at City of Hope Cancer Center, has been a general causation expert for Monsanto, now owned by Bayer, in numerous Roundup trials. In the past two years, Monsanto won nine verdicts in a row until about a month ago, when juries came back with five consecutive awards for plaintiffs totaling more than $2 billion.