Cancer is the leading cause of death worldwide for which effective treatments remain limited. This article aimed to critically review and discuss the potential of targeting cell cycle machineries as a vital tool for cancer treatment. Cyclin dependent kinase (CDK) 4/6 inhibitors were originally approved by the United State Food and Drug Administration (US FDA) for advanced-stage breast cancer treatment. The nearly double-prolonged survival time in patients who received CDK4/6 inhibitors are superior to the conventional chemotherapy or endocrine therapy alone and, thus, these medications have been designated a breakthrough therapy by the US FDA. The requirement of CDK4/6 in the progression of cancer cells, but probably dispensable in normal cells, makes CDK4/6 a popular target for cancer treatment. The effects of CDK4/6 inhibitors in cancer may also involve the tumor microenvironment in which the therapeutic effects are synergistically pronounced. These emerging roles, hence, prompt investigations regarding their therapeutic potential in other cancers, including gastrointestinal cancer. Many preclinical and clinical studies of CDK4/6 inhibitors in gastrointestinal cancers are underway and, as a result, several new potentials are gradually reported. Contrariwise, the primary effect of this drug group is arresting the cell cycle rather than inducing cell death. The efficacy of using CDK4/6 inhibitors as a single regimen in clinical practice is then limited. In this article, the effects of CDK4/6 inhibitors on the progression of gastrointestinal cancers, at both preclinical and clinical levels are reviewed. The future directions for research and the possibility of CDK4/6 inhibitors being "breakthrough therapy" for gastrointestinal cancers are also discussed.Cancer is recognized as a leading cause of death worldwide, whereas effective treatments are underway and yet to be developed for malignancies in certain organs (1). Cancer cells possess malignant hallmarks that differentiate them from their normal counterparts in the same tissues (2). The abnormality at the genetic or epigenetic levels results in cellular transformation and gain the ability to compete and invade their surrounding tissues. Limitless replication is one of the cancer hallmarks (2). Cancer cells gain this ability through various molecular mechanisms, e.g., amplification of proliferation-associated genes, autoactivation of growth signaling pathways, and the mutation of cell cycle inhibitor genes (3). Although these mechanisms benefit cancer cells to survive, it could be a double-sided sword for their survival. The requirement of unnatural overexpression of genes or proteins makes cancer cells dependent on those oncogenes and this dependency becomes their Achilles' heel. 1580