A risk is defined as a combination of the probability of occurrence of harm and the severity of that harm [ 3536 ]. This consent requirement need not imply that embryos are people or that gametes or embryos are research subjects. A cancer scare helped encourage Adult stem cells are bad cell researcher Paul Knoepfler to become an outspoken watchdog over his field. Yet scientists say that the deepest source of public confusion is the fact that the Adult stem cells are bad debate is being waged long before scientists know the answers to many of the most basic questions in stem cell biology, much less which avenues will result in actual treatments. In addition, similar growth regulators and control mechanisms are involved in both cancer and stem cell maintenance [ 39 ].