- Abstract Glycogen storage disease type 1a (GSD 1a), a severe metabolic disorder, is caused by the absence of glucose-6-phosphatase (G6Pase) activity. Diagnosis is currently established by demonstrating the lack of G6Pase activity in the patient's liver specimen. Enzymatic diagnosis cannot be performed in chorionic villi or amniocytes as G6Pase is active only in the liver, kidney, and intestinal mucosa. Recent cloning of the G6Pase gene and subsequent identification of the mutations causing GSD 1a have led to the possibility of performing DNA-based diagnosis in chorionic villus samples (CVS) or amniocytes. Here we report the first DNA-based prenatal diagnosis in two families in whom GSD 1a patients were diagnosed. In one Jewish family with a previously identified R83C mutation, single-stranded conformation polymorphism (SSCP) analysis of the DNA extracted from CVS showed a homozygous R83C mutant pattern. As a result, the pregnancy was terminated and the diagnosis was confirmed on DNA analysis of the aborted fetus. In another family of Arabic extraction in which a V166G mutation has been identified in one of the siblings, SSCP analysis performed on DNA extracted from CVS presented the pattern of a normal control. The pregnancy was carried to term and a healthy baby was born. Thus, once mutations causing the disease are identified, prenatal diagnosis of GSD 1a is possible. SSCP analysis of DNA prepared from CVS is reliable, simple and fast, making it a suitable method for prenatal diagnosis.