Addition of CDK4/6i was associated with increased rates of any grade 3 adverse event (74.8% vs 14.6%), grade 3 or worse neutropenia (43.9% vs 0.7%), diarrhea (37.4% vs 8%), nausea (24.4% vs 10.8%) compared to ET alone, but with lower reported rates of hot flushes (22.5% vs 28.3%) and arthralgia (34.2% vs 41.2%). This evidence concerns the gene CDK4 and neutropenia.