In most patients (ALPS-FAS), ALPS is due to mutations of the Fas gene (FAS), but a small number of patients (ALPS-FASL and ALPS-CASP10) carry mutations in the genes encoding FasL (FASL) or caspase-10 (CASP10), a downstream effector in the Fas/FasL pathway. Here, CASP10 is linked to autoimmune lymphoproliferative syndrome.