Similarly, consistent with most prior studies [20,23,45], the current study reports more prevalent adverse effects in response to CDK4/6 inhibitor-based treatment in patients ≥70 compared to patients <70, leading to more frequent dose reductions, holds, and discontinuations in the older cohort with the most prevalent adverse effects experienced including neutropenia, fatigue, and diarrhea. The gene discussed is CDK4; the disease is neutropenia.