SMA is a neurodegenerative disease in which nerve cells called motor neurons that control movement of voluntary muscles (lower limbs and breathing muscles) are affected. This disease – the leading cause of death from childhood genetic disease – affects 1/6,000 – 1/10,000 live births; with a carrier frequency of 1/40 – 1/60. It is an autosomal recessive disease, so it only occurs if both parents (without symptoms) carry a mutation in the SMN1 gene. A negative result of this test in either partner decreases the risk of having a child with SMA to 1/90,000.