CDKN1B and cancer: β-Carotene treatment arrests a variety of cancer cells in the G0/G1 or G2/M phase: On the one hand, it upregulates CDK inhibitors such as p21 and decreases Cyclin A levels, and on the other hand, it increases intracellular p27Kip1 levels by inhibiting the expression of Skp2 (the protein that drives p27 degradation).