We determined the DNA sequence of the adenylosuccinate lyase (ASL) gene from a 13 year-old female, who showed a reduced ASL enzymatic activity in lymphocytes and red blood cells and suffered from severe psychomotor retardation. The patient was the offspring of a non-consanguineous marriage. She was found to be compound heterozygous for two missense-mutations located on different alleles (C300-G and G1266-T): the first mutation replaces Pro75 by Ala, the second mutation replaces Asp397 by Tyr.