<h4>Methods:</h4> A total of 45 patients with breast cancer able to take abemaciclib (150 mg twice daily) for 2 weeks were evaluated to determine the association among abemaciclib concentrations, adverse events, and ABCB1 1236T>C, 2677G>T/A, 3435C>T, and ABCG2 421C>A gene polymorphisms. This evidence concerns the gene ABCG2 and breast cancer.