The normal cell cycle activation program is exploited during the infection of quiescent B lymphocytes by Epstein-Barr virus. 
B lymphocytes in the peripheral circulation are maintained in a non-proliferative state. Antigen recognition stimulates limited proliferation, whereas infection with Epstein-Barr virus (EBV) results in continual proliferation and the outgrowth of immortal cell lines. Because it is not clear at which point in cell cycle the peripheral B lymphocytes are arrested, we characterized the expression of several cell cycle-associated genes in quiescent and stimulated cells. We show that the expression of four cell genes, cdc-2, cyclin E, CD23, and cyclin D2, are up-regulated approximately 100-fold as a result of EBV-mediated immortalization. Because these genes play a positive role in cell proliferation, we suggest that this regulatory switch contributes to controlling entry into the cell cycle. Transient stimulation of quiescent B lymphocytes with either a cocktail of anti-CD40, anti-IgM, and IL4, or EBV results in the rapid expression of the same four genes, suggesting that, after infection, EBV exploits the normal program of B-lymphocyte cell cycle activation.