One cause of the genetic disease severe combined immunodeficiency (SCID) is a mutation in the ADA gene. This mutation results in a deficiency of the enzyme adenosine deaminase (ADA). Although ADA is found throughout the body, it is especially active in lymphocytes. The absence of functional ADA causes the build-up of toxic metabolites that kill lymphocytes and damage organs. Babies are often diagnosed with SCID by six months old. Treatment can greatly improve the life expectancy of children with SCID. Some treatment options are available. • : Enzyme replacement therapy with recombinant human ADA made by genetically modified (GM) Escherichia coli. Weekly intra-muscular injections are given. Bone marrow transplant if a well-matched donor, such as a close relative, can be found. Gene therapy.