Hunter syndrome is a very rare, inherited genetic disorder caused by a missing or malfunctioning enzyme. Because the body doesn't have enough of the enzyme to break down certain complex molecules, the molecules build up in harmful amounts.

In Hunter syndrome, the buildup of massive amounts of these harmful substances eventually causes permanent, progressive damage affecting appearance, mental development, organ function and physical abilities.

Hunter syndrome appears in children as young as 18 months. It mainly occurs in boys, although very rarely it has been observed in girls.

There's no cure for Hunter syndrome. Treatment of Hunter syndrome involves management of symptoms and complications.