Proposed model of Bcl2/cell cycle signaling. Mono- or multisite Bcl2 phosphorylation reduces intracellular ROS levels through regulation of cellular antioxidant enzymes (ie, SOD and catalase). The decreased intracellular ROS levels induced by Bcl2 result in up-regulation of p27 and down-regulation of Cdk2 activity that are associated with retardation of G1→S cell cycle transition.