Cell Cycle Regulators in Cancer

Cell division is tightly controlled by a machinery of cyclin-dependent kinases (CDKs), cyclins, CDK inhibitors (CKIs), and checkpoint proteins that ensure each phase of the cycle completes accurately before the next begins. The G1/S restriction point — governed by CDK4/CDK6-cyclin D phosphorylation of pRb — is the principal commitment decision in mammalian cells, and it is disrupted in virtually every human cancer through one of several alternative mechanisms. This universality has made cell cycle kinases highly attractive drug targets.

Quick Answer

Proteins controlling when and how cells divide. Disruption of these checkpoints is nearly universal in human cancer. This category links the major genes to their molecular mechanisms, cancer associations, and related pathway pages.

Key Genes in This Category

CDK4

Forms complexes with cyclin D during G1 to phosphorylate pRb, releasing E2F transcription factors for S-phase entry. Amplified in liposarcoma and glioblastoma; CDK4/6 inhibitors palbociclib, ribociclib, and abemaciclib are approved for HR+ breast cancer.

RB1

Encodes pRb, the G1/S restriction point enforcer that binds and represses E2F factors in its hypophosphorylated state. CDK4/6-cyclin D phosphorylation is the irreversible commitment step; virtually every cancer disrupts pRb either directly or through upstream kinase activation.

CDKN2A

Encodes p16INK4a, which competitively inhibits CDK4/CDK6, and p14ARF, which stabilises p53. Deletion removes both the pRb and p53 checkpoint axes simultaneously; among the most commonly deleted loci (~50% of melanomas, ~30% of glioblastomas).

TP53

Transcriptionally activates p21 (CDKN1A) for CDK2 inhibition and G1 arrest, and GADD45 for G2/M arrest following DNA damage. p53-mediated cell cycle arrest buys time for DNA repair before S-phase replication amplifies mutations.

CCND1

Cyclin D1, the regulatory subunit of CDK4/6; its expression is induced by mitogenic signals including RAS/MAPK and Wnt/β-catenin. Amplification in breast cancer, mantle cell lymphoma, and head and neck squamous cancers drives premature G1/S transition.

MYC

Transcriptionally drives cyclin D1 expression and suppresses CDK inhibitors p15, p21, and p27, directly promoting G1/S progression. MYC-driven cells require high CDK4/6 activity and are paradoxically sensitive to CDK inhibition in some models.

ATM

Activates the intra-S and G2/M checkpoints after DNA double-strand breaks through CHK2 and CDC25A degradation, preventing mitosis with unrepaired DNA. ATM loss disables these checkpoints, permitting replication of damaged templates.

AKT1

Phosphorylates and inactivates GSK3β, preventing cyclin D1 degradation and promoting G1/S entry. AKT also phosphorylates p27Kip1 (a CDK inhibitor), targeting it for cytoplasmic sequestration and degradation.

PIK3CA

PI3K activation by PIK3CA mutations drives cyclin D1 expression and CDK activity through AKT/mTOR and directly through RAC1-PAK signalling, providing a parallel route to G1/S bypass independent of direct CDK4 amplification.

MTOR

mTORC1 promotes G1/S progression by driving cap-dependent translation of cyclin D1 and c-MYC. Rapamycin/everolimus inhibits mTORC1 and causes G1 arrest, the primary mechanism of mTOR inhibitor cytostasis in cancer.

Pathways and evidence guides

Move from the gene list into a mechanism map or a focused interpretation guide.

Cell-cycle regulation

Follow cyclin D–CDK4/6, RB and E2F through G1/S.

CDK4/6 inhibitors

Understand mechanism, biomarker context and resistance.

CDKN2A: two suppressors

Compare p16INK4a–RB and p14ARF–p53 control.

RB1 gene profile

Review restriction-point biology and cancer relevance.

Core sources

Authoritative overviews and peer-reviewed sources supporting this category summary.

1What Is Cancer?. National Cancer Institute, 2026. Source
2Hallmarks of Cancer: New Dimensions. Cancer Discovery, 2022. Source
3CDK4/6 inhibitors in cancer therapy: A novel treatement strategy for human cancers. Cancer Letters, 2019. Source

Explore Other Categories

Tumor Suppressor Genes

Proteins that act as molecular brakes on cell division. Loss of both copies unlocks uncontrolled proliferation.

Oncogenes

Gain-of-function mutations in growth-promoting genes that drive continuous cell division, even without normal growth signals.

Apoptosis Regulators

Genes that determine whether a damaged or stressed cell lives or dies. Cancer hijacks these to evade programmed cell death.

DNA Repair Genes

Genes maintaining genome integrity through detection and correction of DNA damage. Their loss drives mutagenesis and cancer predisposition.