Continuation of CDK4/6 inhibition was associated with higher rates of grade 3 and 4 neutropenia (range: 25-40%) and anemia (range: 1.7-11%). In conclusion, switching CDK4/6i and ET conferred a statistically significant improvement of PFS in comparison to ET alone in patients with progression or recurrence on prior CDK4/6i-containing therapy. Here, CDK4 is linked to anemia (phenotype).