Given that many human cancers happen in response to errors in cell cycle regulation and in growth factor dependent intracellular pathways, involvement of cyclin D in cell cycle control and growth factor signaling makes it a possible oncogene. In normal cells overproduction of cyclin D shortens the duration of G1 phase only, and considering the importance of cyclin D in growth factor signaling, defects in its regulation could be responsible for absence of growth regulation in cancer cells. Uncontrolled production of cyclin D affects amounts of cyclin D-Cdk4 complex being formed, which can drive the cell through the G0/S checkpoint, even when the growth factors are not present.