This study will be conducted in 2 sequential parts. The first part will be a prospective, observational study. This will be followed by a small interventional, pilot study involving use of fiber supplement (chia seeds). The study aims to evaluate the relationship between the gut microbiome (a diverse ecosystem of microorganisms that affects your health and well-being) and treatment-related gastrointestinal toxicities in patients with HR+/HER2- breast cancer receiving abemaciclib plus endocrine therapy.

Inclusion Criteria:

• Be willing and able to provide written informed consent for the trial
• Be a male or female subject 18 years of age on day of signing informed consent.
• Histologically proven infiltrating carcinoma of the breast on core needle biopsy that is:

HER2 negative in primary tumour pre-treatment by local pathology assessed according current American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines: In situ hybridization non-amplified (ratio of HER2 to CEP17 < 2.0 or single probe average HER2 gene copy number < 4 signals/cell), OR Immunohistochemistry (IHC) 0 or IHC 1+.

• Estrogen receptor (ER) and progesterone receptors (PR) positive in primary tumour pre-treatment defined as >1% of cells expressing hormonal receptors via IHC analysis by local laboratory assessment.
• Planned to undergo standard of care therapy with abemaciclib and endocrine therapy, in the adjuvant or metastatic setting (Part 1) or the adjuvant setting (Part 2).
• Be willing to provide mandatory stool samples at baseline and on treatment.
• The Eastern Cooperative Oncology Group (ECOG) 0-2

Exclusion Criteria:

• Patients who are pregnant or breast-feeding
• Current use of any investigational agents
• Uncontrolled Ulcerative Colitis, Crohn's Disease, Malabsorption syndrome or any disease significantly affecting gastrointestinal function
• History or current evidence of any condition, therapy, lab abnormality or other circumstance that in the opinion of the investigator might expose the subject to risk by participating in the trial, confound the results of the trial, or interfere with the subject's participation for the full duration of the trial.