In the present study, we demonstrated that heterozygous germline deletion of Gata3 reduces T cell proliferation with induction of p18, but enhances B cell proliferation, and that loss of p18 partially restores Gata3 deficient T cell proliferation but further stimulates Gata3 deficient B cell proliferation, eventually leading to development of B cell lymphomas. The gene discussed is GATA3; the disease is B-cell non-Hodgkin lymphoma.