A mutant gene carried by one out of 40 Jews of Moroccan and Iraqi ancestries that – if found in both parents – may cause serious mental retardation and epilepsy beginning at infancy, has been discovered by a Ben- Gurion University of the Negev team.As the disease is both severe and common, testing for these mutations will probably soon become a routine prenatal genetic screening test in these two populations so that future cases can be prevented, by carriers either not marrying each other or having children together, or by abortion or pre-genetic diagnosis to choose healthy embryos for in-vitro fertilization.Prof. Ohad Birk and doctoral student Orly Agamy of the National Institute for Biotechnology in the Negev at BGU and colleagues at Soroka University Medical Center in Beersheba demonstrated the first human disease associated with a defect in the production of the 21st amino acid, selenocysteine (SEC). Their work was published in the October 8 edition of the American Journal of Human Genetics.They believe that further research will identify other mutations in the same gene as the cause in other ethnic communities of mental retardation with epilepsy. As the disease is progressive, elucidation of its molecular mechanisms might open new venues to treatment, preventing disease progression.The human genetic code is made up of 20 amino acids that are the building blocks of all proteins in the human body. However, in recent years it became apparent that a 21st amino acid exists: selenium entering the body in food is incorporated in the human tissues into what is known as selenocysteine. Healthy expression of seleoproteins is crucial for the function of the neurons, they write.This 21st amino acid is unique in that it is encoded by what is normally a stop codon – a DNA sequence that normally instructs the protein building system to end the chain of amino acids, thus terminating the generated protein. In contrast with most genes, some 25 genes have a unique component that manipulates the stop codon so that instead of terminating the evolving protein chain, it inserts at that point a SEC building block.