cyclin box found in retinoblastoma-associated protein (RB) and similar proteins
RB, also called p105-Rb, pRb, or pp110, is a key regulator of entry into cell division and also acts as a tumor suppressor. It promotes G0-G1 transition when phosphorylated by CDK3/cyclin-C. It also acts as a transcription repressor of E2F1 target genes. RB is directly involved in heterochromatin formation by maintaining overall chromatin structure, especially that of constitutive heterochromatin by stabilizing histone methylation. It recruits and targets histone methyltransferases SUV39H1, KMT5B and KMT5C, leading to epigenetic transcriptional repression. It controls histone H4 'Lys-20' trimethylation. RB contains one cyclin box. The cyclin box is a protein binding domain.