Key takeaway: The pharmacological evaluation highlighted the presence of three risk factors for the development of neutropenia: 1) overexposure to 125 mg/day palbociclib; 2) baseline ANC median value below 3.60 (×103/mm3); 3) low function ABCB1 haplotype. This evidence concerns the gene ABCB1 and Decreased total neutrophil count.