Cyclin-dependent kinase (CDK) 4/6 inhibitors are a class of agents recently introduced in the clinic for the treatment of advanced hormone receptor-positive (HR+) and HER2-negative (HER2-) BC. Palbociclib, ribociclib and abemaciclib have all been approved by the US Food and Drug Administration (FDA) and the European Medicines Agency among other regulatory bodies

Several factors are thought to influence the variability of treatment efficacy and toxicity, including organ function, comedications, hormonal status, body weight, age, comorbidities, etc. Among them, single nucleotide polymorphisms (SNPs) as rs1128503, rs2032583 are attracting huge interest to address the interindividual variability in drug response, whether in terms of efficacy or safety

Inclusion Criteria:

• 1. Adult patients (≥18 years). 2. Confirmed diagnosis with MBC. 3. HR-positive/HER2-negative MBC as ascertained by immunohistochemistry (IHC) or fluorescence in situ hybridization in primary or metastatic tissue samples.

  4. Patients with primary or secondary resistance to endocrine therapy. 5. Patients with Eastern Cooperative Oncology Group (ECOG) 0 or 1. 6. Scheduled to receive Palbociclib in first- or second-line metastatic setting.

Exclusion Criteria:

1. Prior exposure to fulvestrant or everolimus.
2. Patients with uncontrolled brain metastases or symptomatic visceral spread who are at risk of life-threatening complications.
3. Patients refusing to sign the written informed consent.

Hormone receptor-positive HER2-negative metastatic breast cancer patients