Study to Understand Relationship Between Gut Health and Toxicities of Hormonal Breast Cancer Treatment

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 o…