Cdt1 and its inhibitor Geminin are important regulators of replication licensing. In normal cells, a critical balance between these two proteins ensures that firing of each origin along the genome will take place only once per cell cycle. Cdt1 overexpression in cell lines and animals leads to aberrant replication, activates DNA damage checkpoints and predisposes for malignant transformation. Geminin inactivation mimics the effects of Cdt1 overexpression in cells and generates mitotic defects and abnormal chromosome segregation. Aberrant expression of Cdt1 and Geminin is thus linked to DNA replication defects, aneuploidy and genomic instability. These traits are considered integral to precancerous states and essential elements for malignant transformation. Moreover, Cdt1 and Geminin expression is deregulated in human tumor specimens and Cdt1 and Geminin may represent novel markers useful for cancer diagnosis and prognosis.