Sisters Cayetana and Celia, 12, share the same genetic mutation, but only one of them suffers from an ultra-rare disease that turns muscle into bone plates. Celia has practically no symptoms, while Cayetan's muscles are rapidly transforming into bone. Pediatrician Federico Martinón Torres co-directs the medical team that works against the clock to understand why the disease is primed with a sister and respects her twin. "We are always behind the disease," acknowledges the doctor, head of Pediatrics at the University Clinical Hospital of Santiago de Compostela.

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