Janus kinase 1 (JAK1) GOF is responsible for severe atopic dermatitis and hypereosinophilic syndrome characterized by severe eosinophilia with eosinophilic infiltration of the liver and gastrointestinal tract, massive hepatosplenomegaly , autoimmune thyroid disease, and failure to thrive.142. This evidence concerns the gene JAK1 and hypereosinophilic syndrome.