A medical expert testifying about causation in support of the party bearing the burden of proof in a medical malpractice case must be able to express an opinion “to a reasonable medical probability,” which means more likely than not. If the defendant tries to prove that something other than its alleged negligence caused plaintiff’s injury, it bears the burden of proof on that cause, and its expert’s opinion must meet the reasonable medical probability standard. However, if the defendant merely offers expert testimony to dispute plaintiff’s expert’s causation evidence, the reasonable medical probability requirement does not apply, and the defendant’s expert can testify about other possible, even if not probable, causes of the plaintiff’s injury.