We found that MS4a4B negatively regulates T cell proliferation by interfering with cell cycle progression from G0/G1 phase into S-G2/M phases through inhibition of the Cdk2-Rb pathway, and that silence of MS4a4B in thymoma is, at least partially, responsible for the uncontrollable propagation of tumor T cells. This evidence concerns the gene CDK2 and thymoma.