Autosomal homozygous TET2 missense or nonsense in humans results in immunodeficiency, growth impairment and autoimmune lymphoproliferative syndrome (ALPS) like phenotypes of raised proportion of double negative (CD4-CD8-) T-cells (DNTs), raised soluble Fas ligand level, lymphadenopathy, hepatosplenomegaly, autoimmunity and remarkable predisposition to lymphoma, which is reminiscent of phenotypes of TET2 deficient mice (24). The gene discussed is TET2; the disease is Immunodeficiency.