The nonreceptor protein spleen tyrosine kinase (Syk) is a key mediator of signal transduction in a variety of cell types, including B lymphocytes. We show that deregulated Syk activity allows growth factor-independent proliferation and transforms bone marrow-derived pre-B cells that are then able to induce leukemia in mice. Syk-transformed pre-B cells show a characteristic pattern of tyrosine phosphorylation, increased c-Myc expression, and defective differentiation. Treatment of Syk-transformed pre-B cells with a novel Syk-specific inhibitor (R406) reduces tyrosine phosphorylation and c-Myc expression. In addition, R406 treatment removes the developmental block and allows the differentiation of the Syk-transformed pre-B cells into immature B cells. Because R406 treatment also prevents the proliferation of c-Myc-transformed pre-B cells, our data indicate that endogenous Syk kinase activity may be required for the survival of pre-B cells transformed by other oncogenes. Collectively, our data suggest that Syk is a protooncogene involved in the transformation of lymphocytes, thus making Syk a potential target for the treatment of leukemia.