HOX11 is identified from the breakpoint of human T cell acute lymphoblastic leukemias with t(10;14). Since overexpression of HOX11 in T cells caused leukemias in transgenic mice, the endogenous HOX11 may play a role in proliferation and differentiation of T cells. In order to elucidate the role, we examined the expression of Hox11 in normal lymphocytes by a reverse transcriptase-polymerase chain reaction analysis. Two alternatively spliced Hox11 mRNAs were expressed in fetal spleens. However, lymphocytes did not express Hox11 mRNA during differentiation. Furthermore, it was not induced in primary lymphocytes after activation. These results suggest that ectopic expression of HOX11 in T cells is responsible for leukemogenesis.