FAS and autoimmune lymphoproliferative syndrome: Autoimmune lymphoproliferative syndrome (ALPS) is an inborn error of immunity (IEI) due to a defect in the FAS apoptotic pathway, leading to an impairment of lymphocyte homeostasis that clinically brings to chronic lymphoproliferation, autoimmunity (mainly autoimmune cytopenia) and increased risk of lymphoma [1–5].