The FDA has approved Zolgensma, a gene replacement therapy from pharmaceutical company Novartis that treats spinal muscular atrophy, for use in children younger than 2.Why it matters: The treatment attacks a debilitating genetic disease that often kills infants, and it will come with a price tag of more than $2.1 million, making Zolgensma the most expensive drug on the planet.Details: Novartis said it will allow health insurance companies to pay for Zolgensma's $2.1 million price, which does not factor in potential rebates or discounts, over 5 years.That puts Zolgensma's annual list price at $425,000, which Novartis said is less expensive than Spinraza, a competing therapy for spinal muscular atrophy made by Biogen.Between the lines: The Institute for Clinical and Economic Review, a group that evaluates drug pricing and effectiveness, said in a statement that an appropriate all-in price range for Zolgensma would be between $1.1 million and $1.9 million — below what Novartis set. Other ICER estimates say the price should be even lower, between $310,000 and $900,000.Novartis acquired AveXis, the biotech firm that developed this gene therapy, for $8.7 billion last year, so investors want a return for that investment.The bottom line: Zolgensma is emblematic of the new scientific advances that treat people with crippling diseases and of the debate society will have over how it should pay for these types of therapies.Go deeper: The drug pricing debate is stuck in the past