This simplified schematic representation of the cell cycle includes the main molecular participants that are involved in cell-division cycle 25 (CDC25)-mediated cell-cycle progression. Dotted arrows represent known feedback loops, either positive (+) or negative (–), as indicated. It is unclear whether cyclin D–cyclin-dependent kinase 4 (CDK4) is a bona fide substrate of CDC25A. E2F is a transcription factor that was originally identified through its role in transcriptional activation of the adenovirus E2 promoter. P, phosphate; PCNA, proliferating-cell nuclear antigen; Rb, retinoblastoma...