A mouse's immune system generally rejects proteins different from those produced by that mouse. The immune system of a pregnant mouse does not, however, reject the mouse's fetuses, although the fetal tissue a fetus produces as it develops typically contains many such proteins. Some scientists hypothesize that the placenta surrounding the fetus physically blocks the mother's immune system. But others, noting that the placenta produces IDO, an enzyme that suppresses the immune system, hypothesize that IDO plays a crucial role in protecting the fetus.