Target Group: Rb is a tumor suppressor gene which functions as a negative regulator of the cell cycle by interacting with transcription factors including E2F-1, PU.1, ATF-2, UBF, Elf-1 and c-abl. This ability to alter transcription is regulated by phosphorylation catalyzed by the cyclin-dependent protein kinases (cdks). Rb contains at least 16 consensus sequences for cdk phosphorylation, but the significance of all of these sites is unclear. Phosphorylation of threonine 356 is catalyzed by the cdk4 complex Cyclin D-cdk4 and plays a role in mediating growth suppression activity of Rb.