The elucidation of molecular drivers of IEI has important consequences towards the management of these patients since it can rationalize targeted treatment (eg. JAK-inhibition in interferonopathies, TNF-α inhibitors in DADA2, CTLA4 agonists in CTLA4 haploinsufficiency, sirolimus in ALPS, leniolisib in APDS), bone marrow transplant in some cases, provide prognostic information and inform genetic counselling. The gene discussed is CTLA4; the disease is activated PI3K-delta syndrome.