An uncontrolled cell cycle is an obvious marker of tumor cells. The G1‑S phase is an important restriction point in the normal cell cycle, but in cancer cells the restriction function is reduced, leading to uncontrolled cell proliferation. Two cyclin‑dependent kinases (CDKs), CDK4 and CDK6, play a crucial role in the G1‑S phase transition. Inhibitors of CDK4/6 are presently the subjects of numerous studies, and PD 0332991, an inhibitor of CDK4/6, has been used to treat hormone receptor (HR)‑positive, advanced‑stage breast cancer. This inhibitor has also been studied in other cancers, such as lung cancer. In this review, we will discuss the regulation of the normal cell cycle transition from G1 to S phase, the most promising inhibitor of CDK4/6, PD 0332991, as applied in different cancers, and finally we propose a mechanism of acquired resistance as well as the incredible potential for CDK4/6 inhibitors in the treatment of cancer. Briefly, we assert that, going forward, a new treatment pattern for cancer may be a combination therapy with a cell cycle inhibitor and a molecular targeted drug.