When a $2.1 million gene therapy offering the chance of a cure for her daughter’s rare disease won government approval in May, Lauren Sullivan was struck by a sense of “dangerous hope.’’

But after UnitedHealthcare said it would not cover the treatment, Sullivan’s hope has given way to an anxious race against the calendar.

The Food and Drug Administration said the new drug, which works by replacing the defective gene that causes Daryn’s spinal muscular atrophy with a good one, must be administered by age 2. Daryn’s second birthday arrives in early October.

With time running out, the family is caught in a battle over who gets treated with a cutting-edge drug that is also extraordinarily expensive.

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