The invention provides methods of inhibiting growth of tumor cells comprising contacting the cells with a Cdk4 inhibitor. The invention also provides methods of treating patients having, suspected of having, or at a high risk for developing, a cancer, comprising treatment with a Cdk4 inhibitor. The invention also relates to pharmaceutical compositions for treating such patients, wherein the pharmaceutical compositions comprise a Cdk4 inhibitor. The invention further relates to Cdk4 siRNA molecules capable of inhibiting Cdk4 expression or activity.