Study to Evaluate Sodium Phenylbutyrate in Pre-symptomatic Infants With Spinal Muscular Atrophy

Inclusion Criteria: * Laboratory documentation of homozygous absence of SMN1 exon 7. * Confirmation of no more than 3 SMN2 copies for cohort 1; no more than 4 copies for cohort 2. * Family history of affected sibling with SMA type I for cohort 1 and SMA type II for cohort 2. * Age ≤ 3 months, cohort 1; Age ≤ 6 months, cohort 2. * Written informed consent of parents/guardian. * Laboratory results demonstrating normal values for age. Exclusion Criteria: -Evidence of hepatic insufficiency, renal insufficiency, edema with sodium retention, known seizure disorder, urea cycle disorder, cardiac arrhythmia, congenital heart defect, hypertension, significant central nervous system (CNS) impairment, or neurodegenerative or neuromuscular disease other than SMA. History of allergy/sensitivity to sodium phenylbutyrate (NaPB). * Use of NaPB within 30 days of study entry. * Serious illness requiring hospitalization ≤ 14 days prior to study entry. * Use of medications intended for the treatment of SMA including riluzole, valproic acid, hydroxyurea, oral use of albuterol, NaPB, butyrate derivatives, creatine, growth hormone, anabolic steroids, probenecid, oral or parenteral use of corticosteroids at entry, or agents anticipated to increase or decrease muscle strength or agents with presumed histone deacetylase (HDAC) inhibition within 30 days prior to study entry. * Unwillingness to travel for study assessments.