NEW YORK (AP) _ Scientists have identified a defective gene that causes a rare inherited illness resembling Parkinson's disease in people as young as their teens or early 20s.

The finding might eventually help scientists understand what causes Parkinson's disease, which affects more than a half-million Americans.

Parkinson's usually appears after age 50, producing such symptoms as tremors, rigidity and slowing of movement.

This latest research involves a condition called autosomal recessive juvenile parkinsonism.

Japanese scientists report in Thursday's issue of the journal Nature that a gene they dubbed ``parkin'' was mutated in five patients from four families. It's not clear why defects in the gene cause the condition.