A retrospective cohort study of 105 Taiwanese breast cancer patients treated with palbociclib, a targeted CDK4/6 inhibitor, examined the prevalence of neutropenia associated with four SNPs: ABCB1_rs1045642, ABCB1_rs1128503, ERCC1_rs3212986, and ERCC1_rs11615 [69]. Here, CDK4 is linked to neutropenia.