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..