To determine the contribution of CDK2 in driving proliferation of CDK4/6i–resistant MMTV-rtTA/tetO-HER2 tumors, we treated cohorts of tumor-bearing mice with abemaciclib until acquired resistance was observed, and then randomly assigned mice to treatment with vehicle control, continued abemaciclib, INX-315, or their combination. Here, CDK4 is linked to neoplasm.