http://www.google.com/images?q=fila+brasileiro
Fila Brasiliero's , Brazilian Mastiffs: Totally subservient to any household member, great with babies. Totally distrustful of everybody else. Not a dog for the dog park or soccer games. Will not let anyone approach you and will always position themselves between you and anyone else.
Not an attack dog, but a guard dog. Standard procedure without training when you are approached by another is to assume alert guard position, then bark warning, then growl warning, then bite and release without breaking the skin, then bite and hold and use their size to force submission or retreat.
(Not a politically correct breed as they were originally bred to track and capture run-away slaves, without doing any damage to the slave. So bite and release or bite and hold without trying to tear, unless there is resistance.)
No matter how much obedience training, always no matter what, will still maintain alert posture when a household member is in the presence of a stranger; even relatives/friends who are frequent visitors will always remain a stranger.
Some American-bred Fila's are huge, most Brazilian and Mexican bred are smaller and more agile as they are still used as working dogs on cattle ranches.