For example, incomplete penetrance, exemplified by a significant fraction of non-symptomatic carriers, has been described for genetic defects associated with lymphoproliferation and lymphadenopathy, such as activated PI3Kδ syndrome (APDS) (40–43) or FAS-associated autoimmune lymphoproliferative syndrome (ALPS-FAS) (44). This evidence concerns the gene FAS and activated PI3K-delta syndrome.