Inhibition of the cell cycle kinase, cyclin-dependent kinase-4 (Cdk4), is expected to provide an effective method for the treatment of proliferative diseases such as cancer. The pyrido[2,3-d]pyrimidin-7-one template has been identified previously as a privileged structure for the inhibition of ATP-dependent kinases, and good potency against Cdks has been reported for representative examples. Obtaining selectivity for individual Cdk enzymes, particularly Cdk4, has been challenging. Here, we report that the introduction of a methyl substituent at the C-5 position of the pyrido[2,3-d]pyrimidin-7-one template is sufficient to confer excellent selectivity for Cdk4 vs other Cdks and representative tyrosine kinases. Further optimization led to the identification of highly potent and selective inhibitors of Cdk4 that exhibit potent antiproliferative activity against human tumor cells in vitro. The most selective Cdk4 inhibitors were evaluated for antitumor activity against MDA-MB-435 human breast carcinoma xenografts in mice.