Cyclin D1 is a multifunctional protein that activates CDK4 and CDK6, titrates Cip/Kip CDK inhibitors to increase CDK2 activity, and modulates the function of certain transcription factors. To specifically test the importance of cyclin D1-associated kinase activity, we generated "knockin" mice expressing mutant cyclin D1 deficient in activating CDK4/6. The development of several cyclin D1-dependent compartments, including mammary glands, proceeds relatively normally in these animals, demonstrating that cyclin D1-associated kinase activity is largely dispensable for development of these tissues. Strikingly, knockin mice were resistant to breast cancers initiated by ErbB-2. These results demonstrate a differential requirement for cyclin D1-CDK4/6 kinase activity in development versus tumorigenesis and strongly support cyclin D1-dependent kinase activity as a specific therapeutic target in breast cancer.