The present invention relates to the discovery in eukaryotic cells, particularly mammalian cells, of a family of cell-cycle regulatory proteins ("CCR-proteins"). As described herein, this family of proteins includes a polypeptide having an apparent molecular weight of 16 kDa, and a polypeptide having an apparent molecular weight of approximately 15 kDa, each of which can function as an inhibitor of cell-cycle progression, and therefore ultimately of cell growth. The present invention comprises antibodies directed to such CCR-proteins. The present invention is directed to a kit for detecting the level of cyclin-dependent kinase inhibitor p16 gene expression comprising antibodies directed to a p16 protein.