The present invention relates to the use of one or more biomarkers to evaluate the likelihood that a CDK4 inhibitor would produce an anti-cancer effect in a subject. It is based, at least in part, on the discovery that cancer treatment with a CDK4 inhibitor is more effective where treated cancer cells undergo cellular senescence rather than a transient cell cycle arrest, where cellular senescence is associated with decreased MDM2 protein level. Accordingly, in non-limiting embodiments, the present invention provides for methods, compositions, and kits for a companion diagnostic for CDK4 inhibitors, and in particular, to the use of MDM2 expression as a biomarker for the likelihood that a cancer can be successfully treated by CDK4 inhibition.
