Glycerol kinase deficiency (GKD) is an X-linked disorder characterized by elevated plasma or urine glycerol. It may occur as an isolated form caused by mutation of the GK gene alone or as part of a contiguous gene syndrome involving the DAX1 and DMD genes on Xp21.3. Individuals with isolated GKD may be asymptomatic or symptomatic with episodes of vomiting, acidosis and lethargy that may progress to coma or CNS crisis. Phenotypic variability occurs even within families.1 Symptomatic individuals usually present with signs of hypoglycemia, ketoacidosis, and/or seizures. Patients who are symptomatic in childhood may become symptom free. Asymptomatic patients are often identified through hyperlipidemia testing when they are mistaken as having hypertriglyceridemia, as elevated plasma glycerol concentrations can erroneously result in overestimation of plasma triglycerides. Isolated GKD has also been found in children with dysmorphic features and mental retardation; however, at this time, it is not clear whether or not these features are related to GKD alone.2,3 Individuals with GKD as part of the Xp21.3 contiguous gene syndrome also have features of congenital adrenal hypoplasia and/or Duchenne muscular dystrophy.