Cancer is a disease characterized by loss of cellular growth control. As such, it is not surprising that the molecular machinery of the cell cycle is involved in tumorigenesis. Recent discoveries have brought several cell-cycle regulators into sharp focus as factors in human cancer. Among the most conspicuous types of molecule to emerge from ongoing studies in this field are the cyclin-dependent kinase inhibitors such as p16. These molecules have several hallmarks of tumor suppressors and are perfectly positioned to regulate critical decisions in cell growth. The P16 gene appears to be a particularly significant target for mutation in sporadic tumors and in at least one form of hereditary cancer.