Inclusion Criteria:

• Patients age ≥18 years old.
• Patients with a deleterious or suspected deleterious BRCA1 or BRCA2 mutation (germline or somatic) may be enrolled into the study based on either local or central laboratory testing of BRCA status.
• Histologically-confirmed HER2-negative localized breast cancer by core biopsy.
• Primary operable, non-metastatic invasive carcinoma of the breast, confirmed histologically by core biopsy. Fine-needle aspiration is not sufficient. Incisional biopsy is not allowed. In patients with multifocal and/or multicentric, the largest lesion should be measured. Both unilateral and bilateral breast cancer are allowed.
• Primary tumor size ≥1 cm.
• Measurable disease by breast ultrasound and MRI.
• ECOG performance status of 0 or 1.
• Adequate organ function, defined as follows:
  • Note: CBC should be obtained without transfusion or receipt of colony stimulating factors within 4 weeks prior to obtaining a sample.
  • ANC ≥ 1,500/μL;
  • Platelets ≥100,000/μL;
  • Hemoglobin ≥9 g/dL;
  • Serum creatinine ≤1.5 x upper limit of normal (ULN) or calculated creatinine clearance ≥50 mL/min using Cockcroft-Gault equation;
  • Total bilirubin ≤1.5× ULN except in patients with Gilbert・s syndrome. Patients with Gilbert・s syndrome may enroll if direct bilirubin ≤1.5 × ULN of the direct bilirubin;
  • AST and ALT ≤2.5 x ULN.
• Patient must have recovered to Grade 1 toxicity from prior cancer therapy (a patient with Grade 2 neuropathy or Grade 2 alopecia is an exception to this criterion and may qualify for this study).
• Able to take oral medications.
• Patient meets the following criteria:
  • Female patient (of childbearing potential) is not breastfeeding, has a negative serum pregnancy test within 72 hours prior to taking study drug, and agrees to abstain from activities that could result in pregnancy from Screening through 180 days after the last dose of study drug, or is of nonchildbearing potential.
    • Note: A urine pregnancy test may be performed if the serum pregnancy test is not available before dosing.
  • Female patient is of non-childbearing potential (other than medical reasons) as defined by the following:
    • ≥5 years of age and has not had menses for >1 year;
    • Amenorrheic for <2 years without a hysterectomy and oophorectomy and a follicle-stimulating hormone value in the postmenopausal range upon Screening evaluation;
    • Had undergone a hysterectomy, bilateral oophorectomy, or tubal ligation. Documented hysterectomy, oophorectomy, or tubal ligation must be confirmed in the medical records; otherwise the patient must be willing to use 2 adequate barrier methods throughout the study starting from the Screening visit through 180 days after the last dose of study drug. Information must be captured appropriately within the site’s source documents.
  • Male patient agrees to use an effective method of contraception starting with the first dose of study therapy through 120 days after the last dose of study therapy.
    • Note: Abstinence is acceptable if this is the established and preferred contraception for the patient.
• Able to understand the study procedures and agree to participate in the study by providing written informed consent.

Patient Exclusion Criteria:

• Prior anti-cancer therapies for current malignancy.
• Known evidence of distant metastasis. Staging studies are not required.
• The decision to pursue staging studies is at the discretion of the treating clinician, based on the patient’s clinical and pathologic findings consistent with standard guidelines.
• Known hypersensitivity to the components of niraparib components or their formulation excipients.
• Major surgery within 3 weeks of starting the study or patient has not recovered from any effects of any major surgery.
• Poor medical risk due to a serious, uncontrolled medical disorder, non-malignant systemic disease or active, uncontrolled infection. Examples include, but are not limited to, uncontrolled ventricular arrhythmia, recent (within 90 days) myocardial infarction, uncontrolled major seizure disorder, unstable spinal cord compression, superior vena cava syndrome, uncontrolled hypertension, active uncontrolled coagulopathy, bleeding disorder, or any psychiatric disorder that prohibits obtaining informed consent.
• History or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the study, interfere with the patient’s participation for the full duration of the study drug, or is not in the best interest of the patient to participate.
• Patient is pregnant or breastfeeding, or expecting to conceive children within the projected duration of the study drug or within the 180-day period after the last dose of study drug.
• Immunocompromised patients
  • Note: patients with splenectomy are allowed.
• Known active hepatic disease (i.e., Hepatitis B or C).
• Prior treatment with a known PARP inhibitor.
• Other active malignancy that warrants systemic therapy.
• Known history of MDS or AML.