[A negative regulator of the CELL CYCLE that undergoes PHOSPHORYLATION by CYCLIN-DEPENDENT KINASES. It contains a conserved pocket region that binds E2F4 TRANSCRIPTION FACTOR and interacts with viral ONCOPROTEINS such as POLYOMAVIRUS TUMOR ANTIGENS; ADENOVIRUS E1A PROTEINS; and PAPILLOMAVIRUS E7 PROTEINS. ( MSH )] (UMLS (CSP) C0599575) =Amino Acid, Peptide, or Protein; Biologically Active Substance