We have identified an early step common to pathways activated by different forms of intrinsic apoptosis stimuli. It requires de novo synthesis of a novel cyclin, cyclin O, that forms active complexes primarily with Cdk2 upon apoptosis induction in lymphoid cells. Cyclin O expression precedes glucocorticoid and gamma-radiation-induced apoptosis in vivo in mouse thymus and spleen, and its overexpression induces caspase-dependent apoptosis in cultured cells. Knocking down the endogenous expression of cyclin O by shRNA leads to the inhibition of glucocorticoid and DNA damage-induced apoptosis due to a failure in the activation of apical caspases while leaving CD95 death receptor-mediated apoptosis intact. Our data demonstrate that apoptosis induction in lymphoid cells is one of the physiological roles of cyclin O and it does not act by perturbing a normal cellular process such as the cell cycle, the DNA damage checkpoints or transcriptional response to glucocorticoids.