RB1 and thymoma: In primary T cells, CD4+ T cell line and thymoma cells, as confirmed by both overexpression and knockdown approaches, MS4a4B is correlated with increase of cell cycle inhibitors, e.g., p16Ink4a, p21Cip1 and p27Kip1, and decrease of cyclin A and B. Furthermore, overexpression of MS4a4B caused reduction of Cdk2 activity and promoted dephosphorylation of Rb.