If a mutation reduces or eliminates the activity of HMG-CoA lyase, the body is unable to process leucine or make ketones properly. When leucine is not processed normally, chemical byproducts called organic acids can build up and make the blood too acidic (metabolic acidosis). A shortage of ketones can cause blood sugar levels to become dangerously low (hypoglycemia). The effects of metabolic acidosis and hypoglycemia can damage cells, particularly in the brain, resulting in serious illness in children with HMG-CoA lyase deficiency.