CCND1 and breast cancer: Cyclin D1, a well-known oncogene, is upregulated in up to 50% of breast cancers, which promotes G1–S cell cycle progression by forming a complex with cyclin dependent kinases 4/6 (CDK4/6), leading to the Rb phosphorylation and dissociation of transcription factor E2F from the pRb-E2F complex.