FAS and Decreased liver function: He had no history of unexplained fever, liver dysfunction, cytopenia, mosquito bite reactions, or hydroa vacciniforme, features that would suggest the nosologic entity “chronic active EBV.” Based on elevated serum vitamin B12 level, polyclonal hypergammaglobulinemia and 15% DNTC, the diagnosis of ALPS was confirmed by identification of a heterozygous c.784A>T mutation in TNFRSF6, resulting in p.I262F mutation in the intracellular death domain of FAS (GeneDx).