Fumarase deficiency, also known as fumaric aciduria, is a very rare genetic disease characterized by intellectual disability, brain malformation, small head, seizures, and unusual facial features. This is caused by a very low amount of fumarase which is an enzyme essential in cell metabolism and energy production.
Unfortunately, those with this condition often survive for only a few months or years. However, a few victims have been able to live into adulthood. This disease, because it is caused by a rare recessive gene, is normally a rare occurrence in the general population of the planet. However, it is, unfortunately, fairly common in the polygamist communities of the American southwest (the largest known cluster of this condition in the world), due to the the limited gene pool that is caused by marriages between close relatives over several generations. Also see consanguineal marriage.