Signs and symptoms of ADA–SCID typically occur in the first year of life (although about 10% to 15% of people with ADA–SCID have a later onset). Immunodeficiency has the greatest effect on morbidity and mortality, and leads to a high risk of serious and life-threatening recurrent infections. The systemic metabolic defect also causes non-immunological manifestations, including insufficient weight and height gain, cognitive and behavioural problems, and deafness. ADA–SCID has a profound effect on health-related quality of life and, if left untreated, infants die before school age. Quality of life is affected by developmental delay, chronic diarrhoea, failure to thrive, recurrent infections and neurological impairments. People whose condition is untreated must be isolated to reduce the risk of infection. The patient experts highlighted that isolation has a profound effect on both patient and carer quality of life.