What is the best method to confirm that cyclin D1 binds to CDK4?

Co-immunoprecipitation is the most suitable method to confirm the binding of two proteins like cyclin D1 and CDK4, as it involves the use of specific antibodies to isolate one of the proteins along with any proteins interacting together in their native cellular environment. Once the proteins are immunoprecipitated, you can then use techniques like Western blotting to confirm the presence of the other binding partner in the precipitated complex. Affinity chromatography and protein tags are more commonly used for protein purification rather than confirming specific protein-protein interactions. Protein fusion is used to create fusion proteins for various purposes but is not a direct method for confirming protein-protein interactions.