HMGCS2 and hyperinsulinemic hypoglycemia, familial, 4: Here, we present two siblings affected by HMGCS2 deficiency, including the younger sibling who presented with a sepsis‐like condition, neonatal hyperammonemic coma, and an abnormal metabolic screening suggestive of maple syrup urine disease (MSUD); the older sibling was diagnosed through genetic testing of family members.