The neutropenia grade correlated with higher risk, and patients with severe neutropenia (G3–4) in C1D14 or C2D1 had a higher risk of CDK4/6i dose reduction (p < 0.0001; OR 9.1; p < 0.0001; OR 13.9, respectively). The gene discussed is CDK4; the disease is Decreased total neutrophil count.