FAS and autoimmune lymphoproliferative syndrome: However, none of the ALPS patient cases has shown to have germline homozygous or heterozygous mutations in the E163 and E271 of FASLG. Based on our His-tagged FasL and tandem FasL studies, it is evident that both E163 and E271 are highly dominant mutations that totally abolish the Fas cytotoxic signaling.