A method for treating a subject having a cancer selected from the group consisting of liposarcoma, breast cancer, glioma and glioblastoma, comprising, obtaining a sample of the cancer after treatment with a CDK4 inhibitor selected from the group consisting of an ATP-competitive inhibitor of CDK4, pyridopyrimidine compounds, pyrrolopyrimidine compounds, indolocarbazole compounds Palbociclib Isethionate, LEE011, LY2835219, PD0332991, Flavopiridol Hydrochloride, an antisense oligonucleotide that inhibits the expression or activity of CDK4, a shRNA molecule that inhibits the expression or activity of CDK4, a siRNA molecule that inhibits the expression or activity of CDK4 and combinations thereof, and determining, in the sample, the expression level of a MDM2 biomarker, where if the MDM2 biomarker expression level, when compared to a reference standard level, is decreased following treatment with a CDK4 inhibitor, then continuing or resuming treatment of the subject with a therapeutically effective amount of a CDK4 inhibitor.