While none of the cellular markers, such as increased double negative T cells or reduced switched memory B cells, helped to distinguish AI-CVID from FAS-ALPS, increased serum levels of soluble Fas ligand, interleukin (IL) 10, and vitamin B12 allowed a distinction between FAS-ALPS patients and AI-CVID to be made (Rensing-Ehl et al., 2010). This evidence concerns the gene FAS and autoimmune lymphoproliferative syndrome.