About adenosine deaminase deficiency severe combined immunodeficiency ("ADA-SCID") ADA-SCID is a rare inherited disorder of the immune system. The incidence of ADA-SCID is estimated between 1 in every 375,000–660,000 live births, according to literature sources. ADA-SCID is caused by mutations in the gene encoding for the enzyme adenosine deaminase, which result in a severe deficiency in white blood cells and life-threatening infections. In the absence of treatment, ADA-SCID is fatal within the first months of life. Despite currently available treatment options, there remains significant need for therapies that reduce the mortality, morbidity and burden of disease on patients and families.