B99 is a p53-inducible gene whose accumulation upon p53 activation is restricted to late S/G2 cells. Here we have analyzed B99 regulation during the cell cycle in murine cells with or without functional p53. We report that B99 accumulates in late S/G2 phase, is phosphorylated in mitosis, and disappears in G1 phase, regardless of the status of p53. As a complement to this observation, we show that B99 is not induced by p53 in quiescent cells. Therefore, B99 expression is modulated both by cell-cycle regulatory mechanisms and by p53, and p53 can increase the cellular levels of B99 only during the window of the cell cycle when it is normally expressed. On the basis of these observations we rename B99 Gtse-1 (G-two- and S-phase-expressed).