The federal Food and Drug Administration has approved a gene therapy for a rare childhood disorder that is now the most expensive drug on the market. It costs $2.125 million per patient. But for those patients lucky enough to get it, it appears it can save their lives with a one-time treatment...<The medication treats> spinal muscular atrophy (SMA), a rare disorder caused by a defective gene; the illness destroys the nerves that control muscles.