Abemaciclib and Niraparib Before Surgery for the Treatment of Hormone Receptor Positive HER2 Negative Breast Cancer

Inclusion Criteria: * Ability to understand and the willingness to sign a written informed consent document. * Females (regardless of menopausal status), or males * Biopsy proven estrogen receptor (ER) and/or progesterone receptor (PR) positive as defined as ER \>= 1% and/or PR \>= 1% by immunohistochemistry according to American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines for hormone receptor testing * HER2 non-amplified per 2018 ASCO/CAP guidelines, defined as: * Immunohistochemistry (IHC) score 0/1+, or * IHC 2+ and in situ hybridization (ISH) non-amplified with a ratio of HER2 to CEP17 \< 2.0, and if reported, average HER2 gene copy number \< 4 signals/cells; or * ISH non-amplified with a ratio of HER2 to CEP17 \< 2.0, and if reported, average HER2 gene copy number \< 4 signals/cells * Clinical T1-T3, any N, M0 invasive breast cancer, by American Joint Committee on Cancer (AJCC) 8th edition clinical staging with the goal being curative intent surgery to completely excise involved tumor in the breast and the draining lymph nodes * Individuals with unilateral, multi-focal breast cancer (defined as more than one lesion of invasive breast cancer in the same breast separated from the dominant breast lesion by less than 5 cm of radiologically normal breast tissue) are eligible * Participants must be planned for neoadjuvant chemotherapy * Except for allowable endocrine therapy up to 4 weeks prior to study enrollment, participants…