A condition of decreased or absent presence or activity of IL2-inducible t-cell kinase. Deficiency of this protein is associated with lymphoproliferative syndrome 1, an autosomal recessive primary immunodeficiency characterized by onset in early childhood of Epstein-Barr virus (EBV)-associated immune dysregulation, manifest as lymphoma, lymphomatoid granulomatosis, hemophagocytic lymphohistiocytosis, Hodgkin disease, and/or hypogammaglobulinemia.. The disease is lymphoproliferative syndrome 1 (MONDO_0013081). Also known as: ITK deficiency, ITK lymphoproliferative syndrome, LPFS1, lymphoproliferative syndrome caused by mutation in ITK, lymphoproliferative syndrome type 1.