CDKN1B and cancer: p27Kip1 plays a pivotal role in the control of cell proliferation, differentiation, and apoptosis17, 18, 19; it is considered an unusual tumor-suppressor protein, as its deregulated expression in cancer is almost exclusively due to misregulation of its gene expression at the mRNA or the protein level, rather than due to mutations in the p27Kip1 gene.