An exception in which killing another person is permitted is the case of a "rodef" (aggressor), who may be killed in order to save the life of oneself or another. This permits wartime killings and killings in self-defense or to save another from an aggressor in Judaism. Also, abortion is permitted in Jewish law if the unborn fetus is endangering the mother's life, because the fetus is considered to be a "rodef." The "rodef" assertion is also used to permit the separation of conjoined twins when it is likely or even certain that one will die as a result of the operation if this is necessary to save the other.