Mateo is a 4-year-old boy brought in by his parents for increasing clumsiness and frequent falls over the past 18 months. He walked at 20 months and still has not learned to run. Parents note he tires easily and has difficulty climbing stairs. He was in the NICU for 5 days after birth for low muscle tone. Development: first words at 24 months; currently 2–3 word phrases. He is described as a "happy child" who rarely cries. Exam: low-set ears, epicanthal folds, single palmar crease bilaterally, mild hypotonia, and decreased deep tendon reflexes. Previous workup: normal brain MRI, normal metabolic panel. Parents have seen a neurologist ("inconclusive") and a developmental pediatrician ("global delay, etiology unclear").