 ADA-SCID is a rare inherited condition in which there is a change (mutation) in the gene needed to make an enzyme called adenosine deaminase (ADA).As a result, people lack the ADA enzyme.Because ADA is essential for maintaining healthy lymphocytes (white blood cells that fight off infections), the immune system of people with ADA-SCID does not work properly and without effective treatment they rarely survive more than two years.