LAT and lymphoproliferative syndrome: Unexpectedly, a LAT-knockin (LAT-KI) strain of mice in which the sixth tyrosine was mutated to phenylalanine (LATY136F) exhibited a partial block in T-cell maturation, but they later developed a polyclonal lymphoproliferative disorder of CD4 T cells producing massive amounts of T helper type 2 (TH2) cytokines, which caused tissue eosinophilia and maturation of plasma cells secreting huge amounts of IgE and IgG1 immunoglobulins (14, 15).