JAK1 and atopic eczema: Germline JAK1 gain-of-function mutations cause a PID syndrome with autosomal dominant immune dysregulation and hypereosinophilia with eosinophilic infiltration of the gastrointestinal tract, massive hepatosplenomegaly and severe atopic dermatitis that can be successfully treated with ruxolitinib, an oral JAK1/2 inhibitor (40).